If you have a keyboard with the small round PS/2 connector, and you want to connect it to a computer with the older AT standard (large round connector), you will need this female to male PS/2 adapter.
Sandberg Adapter PS/2-keyboard on AT. Gender: Male/Female, Connector(s): PS/2-keyboard, AT, Adapter type: PS/2-keyboard. Color of product: White. Package width: 0.335" (8.5 mm), Package depth: 0.0787" (2 mm), Package height: 0.492" (12.5 mm)