Unicode can be implemented by different character encodings. The most commonly used encodings are UTF-8, UTF-16 and the now-obsolete UCS-2. 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. UCS-2 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 UCS-2, using one 16-bit unit for the characters that were representable in UCS-2 and two 16-bit units (4 × 8 bits) to handle each of the additional characters.
How are two 16-bit units used? 
(4 × 8 bits
UTF-16 extends UCS-2, using one 16-bit unit for the characters that were representable in UCS-2 and two 16-bit units (4 × 8 bits) to handle each of the additional characters.
UTF-sixteen extends UCS-two, using one sixteen-bit unit for the characters that were representable in UCS-two and two sixteen-bit units (four x eight bits) to handle each of the additional characters.
How are two sixteen-bit units used? 