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+## ERC721
+
+<br>
+
+
+To see the difference between ERC20 and ERC721, look at the internal data structure used in ERC721:
+
+
+```
+mapping (uint256 => address) private deedOwner;
+```
+
+<br>
+
+Whereas ERC20 tracks the balances that belong to each owner, with the owner being the primary key of the mapping, 
+ERC721 tracks each deed ID and who owns it, with the deed ID being the primary key of the mapping. 
+
+<br>
+
+
+### The ERC721 contract interface specification
+
+
+```
+interface ERC721 /* is ERC165 */ {
+    event Transfer(address indexed _from, address indexed _to, uint256 _deedId);
+    event Approval(address indexed _owner, address indexed _approved,
+                   uint256 _deedId);
+    event ApprovalForAll(address indexed _owner, address indexed _operator,
+                         bool _approved);
+
+    function balanceOf(address _owner) external view returns (uint256 _balance);
+    function ownerOf(uint256 _deedId) external view returns (address _owner);
+    function transfer(address _to, uint256 _deedId) external payable;
+    function transferFrom(address _from, address _to, uint256 _deedId)
+        external payable;
+    function approve(address _approved, uint256 _deedId) external payable;
+    function setApprovalForAll(address _operator, boolean _approved) payable;
+    function supportsInterface(bytes4 interfaceID) external view returns (bool);
+}
+```