The Commodore 1351 mouse for use with the C64/C128 product line is a small two-button device which is connected to either of the joystick ports on the C64/C128.

The mouse supports two distinct operation modes:
1) Joystick mode.
2) Proportional mode.

Proportional mode is usable with the C64 or the C128, and requires a special machine language driver to yield optimum performance.

Mode selection is determined when the mouse is powered up. If the user depresses the right mouse button when the device is powered up, then the mouse will be in joystick mode.

If the user does not depress the right mouse button when the device is powered up, then the mouse will default to proportional mode.

Joystick mode allows you to utilise the mouse as a joystick when using software which does not support proportional mode.

... The left mouse button is mapped to what would be the fire button on a joystick.

"The Amiga and the Atari ST use an Atari standard DE-9 connector for mice, the same connector that is used for joysticks on the same computers and numerous 8-bit systems, such as the Commodore 64 and the Atari 2600. However, the signals used for mice are different from those used for joysticks. As a result, plugging a mouse into a joystick port causes the "joystick" to continuously move in some direction, even if the mouse stays still, whereas plugging a joystick into a mouse port causes the "mouse" to only be able to move a single pixel in each direction."