Near the bridge/jack.
This is by far the most common place for controls as there isn't a mainstream
electric guitar out there that doesn't use this area for the majority of
their controls, mainly for vol and tone knobs, but also for the pickup
selector on Strats/Teles/super-Strats, etc. In some cases (e.g.,
the SG) you also have an input jack crammed into this area as well.
On the upper bout. Most famously,
Gretsch guitars and LPs place their pickup selector switch in this spot,
but the piezo-equipped specimins of Godin's LGX use this area for the acoustic
vol and EQ for their preamp. Some Gretsch guitars also have a tone
switch here as well.
On the lower bout. This is
easily the least-visited area, design-wise as many shapes don't really
allow enough room to accommodate any controls to speak of. You see
this almost exclusively on hollowbodies such as the master volume knob
on most Gretsch guitars (including the solid body Duo Jet, incidentally)
and the pickup selector switch on many Gibson jazz models such as the L5.
Under the pickguard. I'm not
thinking so much about pickguards on top-routed models like Strats as in
the 3rd dimension: Floating pickguards. Jazz hollowbodies often house
their volume and tone controls here, sometimes discretely. There's
no reason why similar can't be accomplished on a Les Paul.
Note: There's a fourth quadrant, obviously,
but that's ruled out by the ergonomics of having your picking arm in the
way. It's the harest to get to with your hand, but you're going to
bump any controls there with your forearm. I'm not aware of any guitar
design (even the most exotic) that places any components in this area.
Knobs (e.g., vol and tone
pots)
Sliders (e.g., acoustic preamp EQs)
Toggle switches (e.g., Gibson-style
pickup selector switches, mini-toggles for coil taps)
Blade switches (e.g., Strat/Tele
pickup selector switches)
Rotary switches (e.g., PRS selector
switches)
Slider switches (e.g., Jaguar/Jazzmaster
switches)
Momentary switches (e.g., kill switches)
Push-button switches (e.g., series
switches on S-1 equipped Fenders)
Push-pull pots (e.g., coil taps
Schecters)
Note also that many electronic applications
can accommodate either a switch or a potentiometer since the latter are
little more than "on/off" switches with gradations along that continum.