This is a re-post of an article that appeared on Harvey's forum. I'm putting it on my board for easy access.

When mixing in stereo, there are 5 planes of spatial illusion. Level, panning, frequency, spatial perception, and contrast. These five planes are all used to create space in a mix.

Front to back: (Level)
Level gives an element of a mix it's own space. Compression on individual channels helps keep the level so that it doesn't disappear in the mix. A loud instrument will appear forward, or towards the front. A quiet instrument will appear to be back.

Left to right: (Panning)
Panning allows you to give an element of the mix it's own space. For instance putting a guitar part hard right keeps it from washing out the vocal.

Up and down: (Frequency)
Frequency is the use of EQ to boost or cut frequencies that either muddy or clear the mix up. For instance 250Hz-700Hz are fairly muddy frequencies, and if you have too many instruments using this frequency range the mix could be muddy. Everything in an arrangement or mix should have it's own unique fundamental frequency space.

Far and near: (Spatial Perception)
Spatial perception is the use of reverbs, chambers, plates, delays, far mic placement, etc.. to create the illusion of space in the mix. An instrument with allot of reverb can sound like it is placed in a large hall. An instrument or a vocal with a long delay, can sound like it's in the alps. An instrument that's completely dry, will sound like it's in a small carpeted room, right next to you.

Sparse to dense: (Contrast)
Arrangement is the use of muting, and altering the recorded arrangement to create space where it is needed to accent the more dense parts. The use of density to contrast sparse is great for creating the illusion of dynamics in a mix, within minimal dynamic range. The use of a limited dynamic range makes for better listening in more casual environments, where there tends to be external noise.

All 5 of these planes work together to create the illusion of space in a mix. One is no more important than any other in general, although one or two of the planes could prove to be more useful in a given mix. Not all are a requirement for a great mix either. For example, your mix should to be able to break down to mono, and still be a great mix.

Mixerman