A socket includes a plurality of protrusions circumferentially disposed around a circular top opening in a top portion of the socket, whereby upon insertion of a driving (or coupling) shaft of a spanner with square head portion into the square hole in the socket, the square head portion of the driving shaft will be respectively biased or thrusted by the protrusions and then smoothly guided or slid through a sloping surface tapered downwardly radially from the circular opening into the square hole for quickly coupling the driving shaft of the spanner with the square hole in the socket.A socket further includes plural bottom protrusions disposed around a circular bottom opening in a bottom portion of the socket, and eighteen faces formed in a hexagonal bottom hole of the socket for helping a quick coupling of a nut or bolt into the bottom hole of the socket.