Unicode can be implemented by different character encodings. The most commonly used encodings are UTF-8, UTF-16 and the now-obsolete <answer>UCS-2</answer>. UTF-8 uses one byte for any ASCII character, all of which have the same code values in both UTF-8 and ASCII encoding, and up to four bytes for other characters. <answer>UCS-2</answer> uses a 16-bit code unit (two 8-bit bytes) for each character but cannot encode every character in the current Unicode standard. UTF-16 extends <answer>UCS-2</answer>, using one 16-bit unit for the characters that were representable in <answer>UCS-2</answer> and two 16-bit units (4 × 8 bits) to handle each of the additional characters.
What does UTF-16 expand? 
UCS-2