A dimmer switch is one of life's little conveniences that,
once installed, you wonder how you ever got along without!
Any standard single-pole wall switch is a good candidate
for replacement with a dimmer switch – as long as there's
ample room in its electrical box and the light it's
controlling is of the incandescent persuasion.
Don't try to install a dimmer in an undersized box or one
that's already jam-packed with circuit wires, and never
use a dimmer to control a fluorescent light. All types of
dimmer switches have wire leads instead of screw
terminals, so make sure you have the right size wire
connectors on hand.
dimmer has an additional wire lead. This "common" lead is
connected to the common circuit wire. On a three-way
switch, the common circuit wire is the one attached to the
darkest (or copper) screw terminal on the old switch; it
also may be labeled with the word COMMON on the switch
itself. Only one of the pair of three-way switches may be
a dimmer switch; both switches will turn the light fixture
on and off, but only one can control its intensity.