diff --git a/js/id/graph/entity.js b/js/id/graph/entity.js
index 9c26d66e0..ae3520fab 100644
--- a/js/id/graph/entity.js
+++ b/js/id/graph/entity.js
@@ -68,24 +68,8 @@ iD.Entity.prototype = {
         return this._updated && this.osmId() > 0;
     },
 
-    extent: function(resolver) {
-        if (this.type === 'node') {
-            return [this.loc, this.loc];
-        } else if (this.type == 'way') {
-            return resolver.transient(this, 'extent', function() {
-                var extent = [[-Infinity, Infinity], [Infinity, -Infinity]];
-                for (var i = 0, l = this.nodes.length; i < l; i++) {
-                    var node = resolver.entity(this.nodes[i]);
-                    if (node.loc[0] > extent[0][0]) extent[0][0] = node.loc[0];
-                    if (node.loc[0] < extent[1][0]) extent[1][0] = node.loc[0];
-                    if (node.loc[1] < extent[0][1]) extent[0][1] = node.loc[1];
-                    if (node.loc[1] > extent[1][1]) extent[1][1] = node.loc[1];
-                }
-                return extent;
-            });
-        } else {
-            return [[NaN, NaN], [NaN, NaN]];
-        }
+    extent: function() {
+        throw Error("not implemented");
     },
 
     intersects: function(extent, resolver) {
diff --git a/js/id/graph/node.js b/js/id/graph/node.js
index 1a71674d5..3a29cd8e6 100644
--- a/js/id/graph/node.js
+++ b/js/id/graph/node.js
@@ -1,3 +1,7 @@
 iD.Node = iD.Entity.extend({
-    type: "node"
+    type: "node",
+
+    extent: function() {
+        return [this.loc, this.loc];
+    }
 });
diff --git a/js/id/graph/relation.js b/js/id/graph/relation.js
index a4a00dc03..8f65254e8 100644
--- a/js/id/graph/relation.js
+++ b/js/id/graph/relation.js
@@ -1,4 +1,8 @@
 iD.Relation = iD.Entity.extend({
     type: "relation",
-    members: []
+    members: [],
+
+    extent: function() {
+        return [[NaN, NaN], [NaN, NaN]];
+    }
 });
diff --git a/js/id/graph/way.js b/js/id/graph/way.js
index 9c39ba67e..a62afb8e1 100644
--- a/js/id/graph/way.js
+++ b/js/id/graph/way.js
@@ -2,6 +2,20 @@ iD.Way = iD.Entity.extend({
     type: "way",
     nodes: [],
 
+    extent: function(resolver) {
+        return resolver.transient(this, 'extent', function() {
+            var extent = [[-Infinity, Infinity], [Infinity, -Infinity]];
+            for (var i = 0, l = this.nodes.length; i < l; i++) {
+                var node = resolver.entity(this.nodes[i]);
+                if (node.loc[0] > extent[0][0]) extent[0][0] = node.loc[0];
+                if (node.loc[0] < extent[1][0]) extent[1][0] = node.loc[0];
+                if (node.loc[1] < extent[0][1]) extent[0][1] = node.loc[1];
+                if (node.loc[1] > extent[1][1]) extent[1][1] = node.loc[1];
+            }
+            return extent;
+        });
+    },
+
     isOneWay: function() {
         return this.tags.oneway === 'yes';
     },
diff --git a/test/spec/graph/node.js b/test/spec/graph/node.js
index b364dc0e6..c7ce0c406 100644
--- a/test/spec/graph/node.js
+++ b/test/spec/graph/node.js
@@ -21,6 +21,12 @@ describe('iD.Node', function () {
         expect(iD.Node({tags: {foo: 'bar'}}).tags).to.eql({foo: 'bar'});
     });
 
+    describe("#extent", function() {
+        it("returns a point extent", function() {
+            expect(iD.Node({loc: [5, 10]}).extent()).to.eql([[5, 10], [5, 10]]);
+        });
+    });
+
     describe("#intersects", function () {
         it("returns true for a node within the given extent", function () {
             expect(iD.Node({loc: [0, 0]}).intersects([[-180, 90], [180, -90]])).to.equal(true);
diff --git a/test/spec/graph/relation.js b/test/spec/graph/relation.js
index 00e9addd8..dfeeb4e5c 100644
--- a/test/spec/graph/relation.js
+++ b/test/spec/graph/relation.js
@@ -34,4 +34,8 @@ describe('iD.Relation', function () {
     it("sets tags as specified", function () {
         expect(iD.Relation({tags: {foo: 'bar'}}).tags).to.eql({foo: 'bar'});
     });
+
+    describe("#extent", function () {
+        it("returns the minimal extent containing the extents of all members");
+    });
 });
diff --git a/test/spec/graph/way.js b/test/spec/graph/way.js
index bb96a322d..4f64fac67 100644
--- a/test/spec/graph/way.js
+++ b/test/spec/graph/way.js
@@ -35,6 +35,16 @@ describe('iD.Way', function() {
         expect(iD.Way({tags: {foo: 'bar'}}).tags).to.eql({foo: 'bar'});
     });
 
+    describe("#extent", function () {
+        it("returns the minimal extent containing all member nodes", function () {
+            var node1 = iD.Node({loc: [0, 0]}),
+                node2 = iD.Node({loc: [5, 10]}),
+                way   = iD.Way({nodes: [node1.id, node2.id]}),
+                graph = iD.Graph([node1, node2, way]);
+            expect(way.extent(graph)).to.eql([[5, 0], [0, 10]]);
+        });
+    });
+
     describe("#intersects", function () {
         it("returns true for a way with a node within the given extent", function () {
             var node  = iD.Node({loc: [0, 0]}),