Canon EOS-1D Mark II Digial Instruction Manual page 31

Vertical Shooting
For vertical shooting, the vertical grip (camera bottom) has a shutter
button, Main Dial, AF point selection button, AE lock button, Assist
button, and FE lock/multi-spot metering button.
¡Before using the vertical grip controls,
turn on the vertical grip operation on/
off switch.
¡When you are not using the vertical
grip, be sure to turn off the vertical grip
operation on/off switch to prevent
inadvertent operation of the vertical
grip controls.
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