diff --git a/datamodel/low/v3/constants.go b/datamodel/low/v3/constants.go
index f963532..19cd219 100644
--- a/datamodel/low/v3/constants.go
+++ b/datamodel/low/v3/constants.go
@@ -38,4 +38,6 @@ const (
 	PasswordLabel          = "password"
 	ClientCredentialsLabel = "clientCredentials"
 	AuthorizationCodeLabel = "authorizationCode"
+	DescriptionLabel       = "description"
+	URLLabel               = "url"
 )
diff --git a/what-changed/extensions.go b/what-changed/extensions.go
new file mode 100644
index 0000000..c84289e
--- /dev/null
+++ b/what-changed/extensions.go
@@ -0,0 +1,89 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package what_changed
+
+import (
+	"fmt"
+	"github.com/pb33f/libopenapi/datamodel/low"
+	"strings"
+)
+
+type ExtensionChanges struct {
+	PropertyChanges
+}
+
+func CompareExtensions(l, r map[low.KeyReference[string]]low.ValueReference[any]) *ExtensionChanges {
+
+	// look at the original and then look through the new.
+	seenLeft := make(map[string]*low.ValueReference[any])
+	seenRight := make(map[string]*low.ValueReference[any])
+	for i := range l {
+		h := l[i]
+		seenLeft[strings.ToLower(i.Value)] = &h
+	}
+	for i := range r {
+		h := r[i]
+		seenRight[strings.ToLower(i.Value)] = &h
+	}
+
+	var changes []*Change
+	var changeType int
+	for i := range seenLeft {
+		changeType = 0
+		if seenRight[i] == nil {
+			// deleted
+			changeType = PropertyRemoved
+			ctx := CreateContext(seenLeft[i].ValueNode, nil)
+			changes = append(changes, &Change{
+				Context:    ctx,
+				ChangeType: changeType,
+				Property:   i,
+				Original:   fmt.Sprintf("%v", seenLeft[i].Value),
+			})
+
+		}
+		if seenRight[i] != nil {
+			// potentially modified and or moved
+			ctx := CreateContext(seenLeft[i].ValueNode, seenRight[i].ValueNode)
+			if seenLeft[i].Value != seenRight[i].Value {
+				changeType = Modified
+			}
+			if ctx.HasChanged() {
+				if changeType == Modified {
+					changeType = ModifiedAndMoved
+				} else {
+					changeType = Moved
+				}
+			}
+			if changeType != 0 {
+				changes = append(changes, &Change{
+					Context:    ctx,
+					ChangeType: changeType,
+					Property:   i,
+					Original:   fmt.Sprintf("%v", seenLeft[i].Value),
+					New:        fmt.Sprintf("%v", seenRight[i].Value),
+				})
+			}
+		}
+	}
+	for i := range seenRight {
+		if seenLeft[i] == nil {
+			// added
+			ctx := CreateContext(nil, seenRight[i].ValueNode)
+			changes = append(changes, &Change{
+				Context:    ctx,
+				ChangeType: PropertyAdded,
+				Property:   i,
+				New:        fmt.Sprintf("%v", seenRight[i].Value),
+			})
+		}
+	}
+
+	if len(changes) <= 0 {
+		return nil
+	}
+	ex := new(ExtensionChanges)
+	ex.Changes = changes
+	return ex
+}
diff --git a/what-changed/extensions_test.go b/what-changed/extensions_test.go
new file mode 100644
index 0000000..df6afab
--- /dev/null
+++ b/what-changed/extensions_test.go
@@ -0,0 +1,146 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package what_changed
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/low"
+	"github.com/stretchr/testify/assert"
+	"gopkg.in/yaml.v3"
+	"testing"
+)
+
+func TestCompareExtensions(t *testing.T) {
+
+	left := `x-test: 1`
+	right := `x-test: 2`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	lExt := low.ExtractExtensions(lNode.Content[0])
+	rExt := low.ExtractExtensions(rNode.Content[0])
+
+	extChanges := CompareExtensions(lExt, rExt)
+
+	assert.Len(t, extChanges.Changes, 1)
+	assert.Equal(t, Modified, extChanges.Changes[0].ChangeType)
+	assert.Equal(t, "1", extChanges.Changes[0].Original)
+	assert.Equal(t, "2", extChanges.Changes[0].New)
+	assert.False(t, extChanges.Changes[0].Context.HasChanged())
+}
+
+func TestCompareExtensions_Moved(t *testing.T) {
+
+	left := `pizza: pie
+x-test: 1`
+
+	right := `x-test: 1`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	lExt := low.ExtractExtensions(lNode.Content[0])
+	rExt := low.ExtractExtensions(rNode.Content[0])
+
+	extChanges := CompareExtensions(lExt, rExt)
+
+	assert.Len(t, extChanges.Changes, 1)
+	assert.Equal(t, Moved, extChanges.Changes[0].ChangeType)
+	assert.Equal(t, 2, extChanges.Changes[0].Context.OrigLine)
+	assert.Equal(t, 1, extChanges.Changes[0].Context.NewLine)
+	assert.True(t, extChanges.Changes[0].Context.HasChanged())
+}
+
+func TestCompareExtensions_ModifiedAndMoved(t *testing.T) {
+
+	left := `pizza: pie
+x-test: 1`
+
+	right := `x-test: 2`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	lExt := low.ExtractExtensions(lNode.Content[0])
+	rExt := low.ExtractExtensions(rNode.Content[0])
+
+	extChanges := CompareExtensions(lExt, rExt)
+
+	assert.Len(t, extChanges.Changes, 1)
+	assert.Equal(t, ModifiedAndMoved, extChanges.Changes[0].ChangeType)
+	assert.Equal(t, 2, extChanges.Changes[0].Context.OrigLine)
+	assert.Equal(t, 1, extChanges.Changes[0].Context.NewLine)
+	assert.Equal(t, "1", extChanges.Changes[0].Original)
+	assert.Equal(t, "2", extChanges.Changes[0].New)
+	assert.True(t, extChanges.Changes[0].Context.HasChanged())
+}
+
+func TestCompareExtensions_Removed(t *testing.T) {
+
+	left := `pizza: pie
+x-test: 1`
+
+	right := `pizza: pie`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	lExt := low.ExtractExtensions(lNode.Content[0])
+	rExt := low.ExtractExtensions(rNode.Content[0])
+
+	extChanges := CompareExtensions(lExt, rExt)
+
+	assert.Len(t, extChanges.Changes, 1)
+	assert.Equal(t, PropertyRemoved, extChanges.Changes[0].ChangeType)
+	assert.Equal(t, 2, extChanges.Changes[0].Context.OrigLine)
+	assert.Equal(t, -1, extChanges.Changes[0].Context.NewLine)
+	assert.Equal(t, "1", extChanges.Changes[0].Original)
+	assert.True(t, extChanges.Changes[0].Context.HasChanged())
+}
+
+func TestCompareExtensions_Added(t *testing.T) {
+
+	left := `pizza: pie`
+
+	right := `pizza: pie
+x-test: 1`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	lExt := low.ExtractExtensions(lNode.Content[0])
+	rExt := low.ExtractExtensions(rNode.Content[0])
+
+	extChanges := CompareExtensions(lExt, rExt)
+
+	assert.Len(t, extChanges.Changes, 1)
+	assert.Equal(t, PropertyAdded, extChanges.Changes[0].ChangeType)
+	assert.Equal(t, -1, extChanges.Changes[0].Context.OrigLine)
+	assert.Equal(t, 2, extChanges.Changes[0].Context.NewLine)
+	assert.Equal(t, "1", extChanges.Changes[0].New)
+	assert.True(t, extChanges.Changes[0].Context.HasChanged())
+}
+
+func TestCompareExtensions_Identical(t *testing.T) {
+
+	left := `x-test: 1`
+
+	right := `x-test: 1`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	lExt := low.ExtractExtensions(lNode.Content[0])
+	rExt := low.ExtractExtensions(rNode.Content[0])
+
+	extChanges := CompareExtensions(lExt, rExt)
+
+	assert.Nil(t, extChanges)
+}
diff --git a/what-changed/external_docs.go b/what-changed/external_docs.go
new file mode 100644
index 0000000..144ee24
--- /dev/null
+++ b/what-changed/external_docs.go
@@ -0,0 +1,63 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package what_changed
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/low"
+	lowbase "github.com/pb33f/libopenapi/datamodel/low/base"
+	lowv3 "github.com/pb33f/libopenapi/datamodel/low/v3"
+)
+
+type ExternalDocChanges struct {
+	PropertyChanges
+	ExtensionChanges *ExtensionChanges
+}
+
+func CompareExternalDocs(l, r *lowbase.ExternalDoc) *ExternalDocChanges {
+	var changes []*Change
+	changeType := 0
+	if l != nil && r != nil && l.URL.Value != r.URL.Value {
+		changeType = Modified
+		ctx := CreateContext(l.URL.ValueNode, r.URL.ValueNode)
+		if ctx.HasChanged() {
+			changeType = ModifiedAndMoved
+		}
+		changes = append(changes, &Change{
+			Context:    ctx,
+			ChangeType: changeType,
+			Property:   lowv3.URLLabel,
+			Original:   l.URL.Value,
+			New:        r.URL.Value,
+		})
+	}
+	if l != nil && r != nil && l.Description.Value != r.Description.Value {
+		changeType = Modified
+		ctx := CreateContext(l.Description.ValueNode, r.Description.ValueNode)
+		if ctx.HasChanged() {
+			changeType = ModifiedAndMoved
+		}
+		changes = append(changes, &Change{
+			Context:    ctx,
+			ChangeType: changeType,
+			Property:   lowv3.DescriptionLabel,
+			Original:   l.Description.Value,
+			New:        r.Description.Value,
+		})
+	}
+	if changeType == 0 {
+		// no change, return nothing.
+		return nil
+	}
+	dc := new(ExternalDocChanges)
+	dc.Changes = changes
+	var lExt, rExt map[low.KeyReference[string]]low.ValueReference[any]
+	if l != nil && len(l.Extensions) > 0 {
+		lExt = l.Extensions
+	}
+	if r != nil && len(r.Extensions) > 0 {
+		rExt = r.Extensions
+	}
+	dc.ExtensionChanges = CompareExtensions(lExt, rExt)
+	return dc
+}
diff --git a/what-changed/external_docs_test.go b/what-changed/external_docs_test.go
new file mode 100644
index 0000000..a9ba48b
--- /dev/null
+++ b/what-changed/external_docs_test.go
@@ -0,0 +1,154 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package what_changed
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/low"
+	lowbase "github.com/pb33f/libopenapi/datamodel/low/base"
+	lowv3 "github.com/pb33f/libopenapi/datamodel/low/v3"
+	"github.com/stretchr/testify/assert"
+	"gopkg.in/yaml.v3"
+	"testing"
+)
+
+func TestCompareExternalDocs(t *testing.T) {
+
+	left := `url: https://pb33f.io
+description: this is a test
+x-testing: hello`
+
+	right := `url: https://quobix.com
+description: this is another test
+x-testing: hiya!`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	// create low level objects
+	var lDoc lowbase.ExternalDoc
+	var rDoc lowbase.ExternalDoc
+	_ = low.BuildModel(&lNode, &lDoc)
+	_ = low.BuildModel(&rNode, &rDoc)
+	_ = lDoc.Build(lNode.Content[0], nil)
+	_ = rDoc.Build(rNode.Content[0], nil)
+
+	// compare.
+	extChanges := CompareExternalDocs(&lDoc, &rDoc)
+	assert.Len(t, extChanges.ExtensionChanges.Changes, 1)
+	assert.Len(t, extChanges.Changes, 2)
+
+	// validate property changes
+	urlChange := extChanges.Changes[0]
+	assert.Equal(t, Modified, urlChange.ChangeType)
+	assert.False(t, urlChange.Context.HasChanged())
+	assert.Equal(t, "https://pb33f.io", urlChange.Original)
+	assert.Equal(t, "https://quobix.com", urlChange.New)
+	assert.Equal(t, 1, urlChange.Context.OrigLine)
+	assert.Equal(t, lowv3.URLLabel, urlChange.Property)
+
+	descChange := extChanges.Changes[1]
+	assert.Equal(t, Modified, descChange.ChangeType)
+	assert.False(t, descChange.Context.HasChanged())
+	assert.Equal(t, "this is another test", descChange.New)
+	assert.Equal(t, "this is a test", descChange.Original)
+	assert.Equal(t, 2, descChange.Context.OrigLine)
+	assert.Equal(t, 14, descChange.Context.OrigCol)
+
+	// validate extensions
+	extChange := extChanges.ExtensionChanges.Changes[0]
+	assert.Equal(t, Modified, extChange.ChangeType)
+	assert.False(t, extChange.Context.HasChanged())
+	assert.Equal(t, "hiya!", extChange.New)
+	assert.Equal(t, "hello", extChange.Original)
+	assert.Equal(t, 3, extChange.Context.OrigLine)
+	assert.Equal(t, 12, extChange.Context.OrigCol)
+
+}
+
+func TestCompareExternalDocs_Moved(t *testing.T) {
+
+	left := `url: https://pb33f.io
+description: this is a test
+x-testing: hello`
+
+	right := `description: this is another test
+x-testing: hiya!
+url: https://quobix.com`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	// create low level objects
+	var lDoc lowbase.ExternalDoc
+	var rDoc lowbase.ExternalDoc
+	_ = low.BuildModel(&lNode, &lDoc)
+	_ = low.BuildModel(&rNode, &rDoc)
+	_ = lDoc.Build(lNode.Content[0], nil)
+	_ = rDoc.Build(rNode.Content[0], nil)
+
+	// compare.
+	extChanges := CompareExternalDocs(&lDoc, &rDoc)
+	assert.Len(t, extChanges.ExtensionChanges.Changes, 1)
+	assert.Len(t, extChanges.Changes, 2)
+
+	// validate property changes
+	urlChange := extChanges.Changes[0]
+	assert.Equal(t, ModifiedAndMoved, urlChange.ChangeType)
+	assert.True(t, urlChange.Context.HasChanged())
+	assert.Equal(t, "https://pb33f.io", urlChange.Original)
+	assert.Equal(t, "https://quobix.com", urlChange.New)
+	assert.Equal(t, 1, urlChange.Context.OrigLine)
+	assert.Equal(t, 3, urlChange.Context.NewLine)
+	assert.Equal(t, lowv3.URLLabel, urlChange.Property)
+
+	descChange := extChanges.Changes[1]
+	assert.Equal(t, ModifiedAndMoved, descChange.ChangeType)
+	assert.True(t, descChange.Context.HasChanged())
+	assert.Equal(t, "this is another test", descChange.New)
+	assert.Equal(t, "this is a test", descChange.Original)
+	assert.Equal(t, 2, descChange.Context.OrigLine)
+	assert.Equal(t, 14, descChange.Context.OrigCol)
+	assert.Equal(t, 1, descChange.Context.NewLine)
+	assert.Equal(t, 14, descChange.Context.NewCol)
+
+	// validate extensions
+	extChange := extChanges.ExtensionChanges.Changes[0]
+	assert.Equal(t, ModifiedAndMoved, extChange.ChangeType)
+	assert.True(t, extChange.Context.HasChanged())
+	assert.Equal(t, "hiya!", extChange.New)
+	assert.Equal(t, "hello", extChange.Original)
+	assert.Equal(t, 3, extChange.Context.OrigLine)
+	assert.Equal(t, 12, extChange.Context.OrigCol)
+	assert.Equal(t, 2, extChange.Context.NewLine)
+	assert.Equal(t, 12, extChange.Context.NewCol)
+}
+
+func TestCompareExternalDocs_Identical(t *testing.T) {
+
+	left := `url: https://pb33f.io
+description: this is a test
+x-testing: hello`
+
+	right := `url: https://pb33f.io
+description: this is a test
+x-testing: hello`
+
+	var lNode, rNode yaml.Node
+	_ = yaml.Unmarshal([]byte(left), &lNode)
+	_ = yaml.Unmarshal([]byte(right), &rNode)
+
+	// create low level objects
+	var lDoc lowbase.ExternalDoc
+	var rDoc lowbase.ExternalDoc
+	_ = low.BuildModel(&lNode, &lDoc)
+	_ = low.BuildModel(&rNode, &rDoc)
+	_ = lDoc.Build(lNode.Content[0], nil)
+	_ = rDoc.Build(rNode.Content[0], nil)
+
+	// compare.
+	extChanges := CompareExternalDocs(&lDoc, &rDoc)
+	assert.Nil(t, extChanges)
+}
diff --git a/what-changed/what_changed.go b/what-changed/what_changed.go
index 0a95109..5237ee9 100644
--- a/what-changed/what_changed.go
+++ b/what-changed/what_changed.go
@@ -2,3 +2,176 @@
 // SPDX-License-Identifier: MIT
 
 package what_changed
+
+import (
+	"gopkg.in/yaml.v3"
+)
+
+const (
+	Modified = iota + 1
+	PropertyAdded
+	ObjectAdded
+	ObjectRemoved
+	PropertyRemoved
+	Moved
+	ModifiedAndMoved
+)
+
+type WhatChanged struct {
+	Added        int
+	Removed      int
+	Modified     int
+	Moved        int
+	TotalChanges int
+	Changes      *Changes
+}
+
+type ChangeContext struct {
+	OrigLine int
+	OrigCol  int
+	NewLine  int
+	NewCol   int
+}
+
+func (c *ChangeContext) HasChanged() bool {
+	return c.NewLine != c.OrigLine || c.NewCol != c.OrigCol
+}
+
+type Change struct {
+	Context    *ChangeContext
+	ChangeType int
+	Property   string
+	Original   string
+	New        string
+}
+
+type PropertyChanges struct {
+	Changes []*Change
+}
+
+type TagChanges struct {
+	PropertyChanges
+	ExternalDocs *ExternalDocChanges
+}
+
+type Changes struct {
+	TagChanges *TagChanges
+}
+
+//func WhatChangedBetweenDocuments(leftDocument, rightDocument *lowv3.Document) *WhatChanged {
+//
+//	// compare tags
+//	//leftTags := leftDocument.Tags.Value
+//	//rightTags := rightDocument.Tags.Value
+//
+//	return nil
+//}
+
+func CreateContext(l, r *yaml.Node) *ChangeContext {
+	ctx := new(ChangeContext)
+	if l != nil {
+		ctx.OrigLine = l.Line
+		ctx.OrigCol = l.Column
+	} else {
+		ctx.OrigLine = -1
+		ctx.OrigCol = -1
+	}
+	if r != nil {
+		ctx.NewLine = r.Line
+		ctx.NewCol = r.Column
+	} else {
+		ctx.NewLine = -1
+		ctx.NewCol = -1
+	}
+	return ctx
+}
+
+//
+//func compareTags(l, r []low.ValueReference[*lowbase.Tag]) *TagChanges {
+//
+//	tc := new(TagChanges)
+//
+//	// look at the original and then look through the new.
+//	seenLeft := make(map[string]*low.ValueReference[*lowbase.Tag])
+//	seenRight := make(map[string]*low.ValueReference[*lowbase.Tag])
+//	for i := range l {
+//		seenLeft[strings.ToLower(l[i].Value.Name.Value)] = &l[i]
+//	}
+//	for i := range r {
+//		seenRight[strings.ToLower(l[i].Value.Name.Value)] = &l[i]
+//	}
+//
+//	for i := range seenLeft {
+//		if seenRight[i] == nil {
+//			// deleted
+//			//ctx := CreateContext(seenLeft[i].ValueNode, nil)
+//			//tc.Changes =
+//
+//		}
+//		if seenRight[i] != nil {
+//
+//			// potentially modified and or moved
+//		}
+//	}
+//
+//	for i := range seenRight {
+//		if seenLeft[i] == nil {
+//			// added
+//		}
+//	}
+//
+//	for i := range r {
+//		// if we find a match
+//		t := r[i]
+//		name := r[i].Value.Name.Value
+//		found := seenLeft[strings.ToLower(name)]
+//		if found.Value != nil {
+//
+//			// check values
+//			if found.Value.Description.Value != t.Value.Description.Value {
+//				ctx := CreateContext(found.ValueNode, t.ValueNode)
+//				changeType := Modified
+//				if ctx.HasChanged() {
+//					changeType = ModifiedAndMoved
+//				}
+//				tc.Changes = append(tc.Changes, &Change{
+//					Context:    ctx,
+//					ChangeType: changeType,
+//					Property:   lowv3.DescriptionLabel,
+//					Original:   found.Value.Description.Value,
+//					New:        t.Value.Description.Value,
+//				})
+//			}
+//
+//		} else {
+//
+//			// new stuff
+//
+//		}
+//
+//	}
+//
+//	// more tags in right hand-side
+//	if len(r) > len(l) {
+//
+//	}
+//
+//	// less tags in right hand-side
+//	if len(r) < len(l) {
+//
+//	}
+//
+//	//for i := range a {
+//	//	eq, l, c := comparePositions(a)
+//	//
+//	//}
+//
+//	return nil
+//}
+//
+//func comparePositions(left, right *yaml.Node) (bool, int, int) {
+//	if left.Line == right.Line && left.Column == right.Column {
+//		return true, 0, 0
+//	}
+//	return false, right.Line, right.Column
+//}
diff --git a/what-changed/what_changed_test.go b/what-changed/what_changed_test.go
new file mode 100644
index 0000000..6e78fbd
--- /dev/null
+++ b/what-changed/what_changed_test.go
@@ -0,0 +1,5 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package what_changed
+