Complex sentence: Although a key signature may be written using any combination of sharp and flat symbols, about a dozen diatonic key signatures are by far the most common, and their use is assumed in much of this article.
Split into simpler sentences: Although a key signature may be written using any combination of sharp and flat symbols, only a small number of different key signatures are commonly used. These are the diatonic key signatures, and their use is assumed in much of this article.