Number 3 Robertson Scrulox / square drive screwdriver.
BLACK HANDLE - the colour indicates the size, so you can quickly identify the driver you need from the rest of the stuff in your toolbox.
Very popular with tradesmen.
This screwdriver suits number 12 and 14 screws (larger type).
Robertson screwdrivers are easy to use one-handed, because the tapered socket tends to retain the screw, even if it is shaken.