I'm sure many of you are aware of the concept of dropping the mids on acoustic guitar to make room for vocals, or pulling back the low end a bit if it interferes with bass. I kept thinking that's kind of a broad approach, and there should be a way to accomplish a similar result less invasively.
 
Long story short, I think I've found it. I inserted a Sonitus EQ in the rhythm guitar's FX bin; the song was in the key of A. So, I created six notches using the sharpest Q possible at the following frequencies: 110, 220, 440, 880, 1760, and 3520. Pulling back on the gain for each band took out the "meat" from the guitar, giving it a somewhat thinner but extremely balanced sound. It fit more transparently into the track, and its "key of A-ness" stayed out of the way of other instruments.
 
I guess that this work also work with something like piano, if you wanted to thin it out somewhat but leave the guitar sound as is because you want to emphasize it.
 
Anyway, I thought you might find this interesting. It really does seem to open up some more "space" when mixing.

Cool, now lets take it to the next level and have the EQ automated to follow the chord changes!! 
I've gotta try this. Thanks for the post Craig. It is timely as I'm just in the final mix stage on a acoustic album and there's the usual fight for frequency with the Mandolin, guitar and banjo. 
 
http://www.soundclick.com/player/single_player.cfm?songid=12782255&q=hi&newref=1
 
 

Key of A Major:
I:    A     110...
      C#   138.6...
      E     164.8...
 
IV:  D    146.8...
      F#   185...
      A     110...
 
V:   E     164.8...
      G#   207.7...
      B     123.5...
 
 
fixed

If one needed a tool to visualize the collisions, MeldaProduction MMultiAnalyzer is on sale for ~$40 for the next couple of days.

Interesting technique.  Maybe Cake needs to implement a chordal EQ, smart so you set the key and changes and the eq proportionally follows along.
 
@

I'm not sure it would be necessary to go to too great lengths with all this. In the prelim tests I did, I played various chord progressions and just dealing with the tonic seemed to make a sufficient difference. However, the reason why I post these ideas/works-in-progress is in case others want to pursue, and add their own expertise/experiences.
 
I also found that the two highest notes aren't really all that important, it's the lowest four that matter.

Anderton
I'm not sure it would be necessary to go to too great lengths with all this. In the prelim tests I did, I played various chord progressions and just dealing with the tonic seemed to make a sufficient difference. However, the reason why I post these ideas/works-in-progress is in case others want to pursue, and add their own expertise/experiences.
 
I also found that the two highest notes aren't really all that important, it's the lowest four that matter.

I agree it's not that important to go through all the automation of the EQ. I do agree with the original premise of the test. I think subtractive would be better though. Either way it does have it's drawbacks as even if you hit a single F#, there are a multitude of overtones and those can add the flavor of the note. Still, an interesting idea as you could subtract the frequencies just above or below the target note. Say and A 440. Drop the frequencies below 420 or above 460. Even then it depends on how it sounds. I think it's a good idea to EQ by key but all this talk about automating the EQ , whatever, is overthinking things. I would worry about the space for each instrument in relation to the key.

There is a free course, Play with your Music, and they say that  even though you hit the note A 440, all frequencies sound out. It's just which frequencies prevail.

Guitarpima
 
I agree it's not that important to go through all the automation of the EQ. I do agree with the original premise of the test. I think subtractive would be better though.

 
Actually the original premise was subtractive - six notches, although now I think four is sufficient. The idea is to subtract the fundamental and its even harmonics only, but much of the note remains regardless. The purpose is really to "thin out" one instrument so it doesn't compete with other instruments, which  you don't thin out. It also prevents build-ups at specific frequencies, which may or may not be helpful...doing so does allow a somewhat higher average level without resorting to dynamics.

Interesting, I must try it out.
 

As well as rhythm guitar, this idea might work well when applied to synth pads too.