diff --git a/js/id/graph/graph.js b/js/id/graph/graph.js
index 120fa2fe7..3eea6a59a 100644
--- a/js/id/graph/graph.js
+++ b/js/id/graph/graph.js
@@ -1,13 +1,15 @@
-iD.Graph = function(entities, mutable) {
-    if (!(this instanceof iD.Graph)) return new iD.Graph(entities, mutable);
+iD.Graph = function(other, mutable) {
+    if (!(this instanceof iD.Graph)) return new iD.Graph(other, mutable);
 
-    if (_.isArray(entities)) {
-        this.entities = {};
-        for (var i = 0; i < entities.length; i++) {
-            this.entities[entities[i].id] = entities[i];
+    if (_.isArray(other)) {
+        this.entities = Object.create({});
+        for (var i = 0; i < other.length; i++) {
+            this.entities[other[i].id] = other[i];
         }
+    } else if (other instanceof iD.Graph) {
+        this.rebase(other.base(), other.entities);
     } else {
-        this.entities = entities || {};
+        this.entities = Object.create(other || {});
     }
 
     this.transients = {};
@@ -97,10 +99,18 @@ iD.Graph.prototype = {
         return (this._childNodes[entity.id] = nodes);
     },
 
-    merge: function(graph) {
-        return this.update(function () {
-            _.defaults(this.entities, graph.entities);
-        });
+    base: function() {
+        return Object.getPrototypeOf ?
+            Object.getPrototypeOf(this.entities) :
+            this.entities.__proto__;
+    },
+
+    // Unlike other graph methods, rebase mutates in place. This is because it
+    // is used only during the history operation that merges newly downloaded
+    // data into each state. To external consumers, it should appear as if the
+    // graph always contained the newly downloaded data.
+    rebase: function(base, entities) {
+        this.entities = _.assign(Object.create(base), entities || this.entities);
     },
 
     replace: function(entity) {
@@ -120,7 +130,7 @@ iD.Graph.prototype = {
     },
 
     update: function() {
-        var graph = this.frozen ? iD.Graph(_.clone(this.entities), true) : this;
+        var graph = this.frozen ? iD.Graph(this, true) : this;
 
         for (var i = 0; i < arguments.length; i++) {
             arguments[i].call(graph, graph);
@@ -133,7 +143,6 @@ iD.Graph.prototype = {
         this.frozen = true;
 
         if (iD.debug) {
-            Object.freeze(this);
             Object.freeze(this.entities);
         }
 
diff --git a/js/id/graph/history.js b/js/id/graph/history.js
index 29b652554..c9e3e19f5 100644
--- a/js/id/graph/history.js
+++ b/js/id/graph/history.js
@@ -30,8 +30,12 @@ iD.History = function() {
         },
 
         merge: function (graph) {
-            for (var i = 0; i < stack.length; i++) {
-                stack[i].graph = stack[i].graph.merge(graph);
+            var base = stack[0].graph.base();
+
+            _.defaults(base, graph.entities);
+
+            for (var i = 1; i < stack.length; i++) {
+                stack[i].graph.rebase(base);
             }
         },
 
diff --git a/test/spec/graph/graph.js b/test/spec/graph/graph.js
index 25382e7c9..fa83a3a03 100644
--- a/test/spec/graph/graph.js
+++ b/test/spec/graph/graph.js
@@ -1,25 +1,110 @@
 describe('iD.Graph', function() {
-    it("can be constructed with an entities Object", function () {
-        var entity = iD.Entity(),
-            graph = iD.Graph({'n-1': entity});
-        expect(graph.entity('n-1')).to.equal(entity);
-    });
-
-    it("can be constructed with an entities Array", function () {
-        var entity = iD.Entity(),
-            graph = iD.Graph([entity]);
-        expect(graph.entity(entity.id)).to.equal(entity);
-    });
-
-    if (iD.debug) {
-        it("is frozen", function () {
-            expect(Object.isFrozen(iD.Graph())).to.be.true;
+    describe("constructor", function () {
+        it("accepts an entities Object", function () {
+            var entity = iD.Entity(),
+                graph = iD.Graph({'n-1': entity});
+            expect(graph.entity('n-1')).to.equal(entity);
         });
 
-        it("freezes entities", function () {
-            expect(Object.isFrozen(iD.Graph().entities)).to.be.true;
+        it("accepts an entities Array", function () {
+            var entity = iD.Entity(),
+                graph = iD.Graph([entity]);
+            expect(graph.entity(entity.id)).to.equal(entity);
         });
-    }
+
+        it("accepts a Graph", function () {
+            var entity = iD.Entity(),
+                graph = iD.Graph(iD.Graph([entity]));
+            expect(graph.entity(entity.id)).to.equal(entity);
+        });
+
+        it("copies other's entities", function () {
+            var entity = iD.Entity(),
+                base   = iD.Graph([entity]),
+                graph  = iD.Graph(base);
+            expect(graph.entities).not.to.equal(base.entities);
+        });
+
+        it("rebases on other's base", function () {
+            var base   = iD.Graph(),
+                graph  = iD.Graph(base);
+            expect(graph.base()).to.equal(base.base());
+        });
+
+        it("freezes by default", function () {
+            expect(iD.Graph().frozen).to.be.true;
+        });
+
+        it("remains mutable if passed true as second argument", function () {
+            expect(iD.Graph([], true).frozen).not.to.be.true;
+        });
+    });
+
+    describe("#freeze", function () {
+        it("sets the frozen flag", function () {
+            expect(iD.Graph([], true).freeze().frozen).to.be.true;
+        });
+
+        if (iD.debug) {
+            it("freezes entities", function () {
+                expect(Object.isFrozen(iD.Graph().entities)).to.be.true;
+            });
+        }
+    });
+
+    describe("#rebase", function () {
+        it("preserves existing entities", function () {
+            var node = iD.Node({id: 'n'}),
+                graph = iD.Graph([node]);
+            graph.rebase({});
+            expect(graph.entity('n')).to.equal(node);
+        });
+
+        it("includes new entities", function () {
+            var node = iD.Node({id: 'n'}),
+                graph = iD.Graph();
+            graph.rebase({'n': node});
+            expect(graph.entity('n')).to.equal(node);
+        });
+
+        it("gives precedence to existing entities", function () {
+            var a = iD.Node({id: 'n'}),
+                b = iD.Node({id: 'n'}),
+                graph = iD.Graph([a]);
+            graph.rebase({'n': b});
+            expect(graph.entity('n')).to.equal(a);
+        });
+
+        it("inherits entities from base prototypally", function () {
+            var graph = iD.Graph();
+            graph.rebase({'n': iD.Node()});
+            expect(graph.entities).not.to.have.ownProperty('n');
+        });
+
+        xit("updates parentWays", function () {
+            var n = iD.Node({id: 'n'}),
+                w1 = iD.Way({id: 'w1', nodes: ['n']}),
+                w2 = iD.Way({id: 'w2', nodes: ['n']}),
+                graph = iD.Graph([n, w1]);
+
+            graph.parentWays(n);
+            graph.rebase({'w2': w2});
+
+            expect(graph.parentWays(n)).to.eql([w1, w2]);
+        });
+
+        xit("updates parentRelations", function () {
+            var n = iD.Node({id: 'n'}),
+                r1 = iD.Relation({id: 'r1', members: [{id: 'n'}]}),
+                r2 = iD.Relation({id: 'r2', members: [{id: 'n'}]}),
+                graph = iD.Graph([n, r1]);
+
+            graph.parentRelations(n);
+            graph.rebase({'r2': r2});
+
+            expect(graph.parentRelations(n)).to.eql([r1, r2]);
+        });
+    });
 
     describe("#remove", function () {
         it("returns a new graph", function () {