Overview of Four-Part Writing

I've long thought that writing for four parts - soprano, alto, tenor, and bass (SATB) - could be thought of in terms of sardines. Imagine a choir of sardines swimming in different levels of the ocean. The soprano sardines are at the top, with the alto, tenor, and bass ones in respective layers beneath (vocal ranges). They might be tighly packed - like they are in sardine cans - in close structure, or more widely spaced (structure). Just as the SATB human voices are limited to four pitch ranges, the four layers of fish would be confined to their particular depths (spacing). I imagine that real sardines swim in schools, synchronized, but our sophisticated sardine choir prides itself on independence. Thus, if one depth of sardines parrots another, they get miffed, because "following the leader" is frowned upon (parallelsm). Sometimes a layer of sardines happens upon certain tendencies to move this way or that (tendency tones). And finally, there's occasion for two layers of sardines to double up and temporarily clone themselves (doubling). All of this might sound - ahem - fishy, but personally I think the metaphor will help you write four-part music swimmingly!