Figure shows the four exchanges required to close a connection.  The first step shown is when client A is ready to close the connection therefore it sends a segment with the FIN flag set and it is received by client B.  Step two has client B sending an ACK to client A.  Step three is also from client B and it sends a FIN to client A to terminate the server to client session and the client receives the FIN.  The final step is when client A responds to client B by sending an ACK to client B.