As dscussed in the previous topic, scales represent a pitch collection that centralizes around a tonic pitch. Because we can transpose these pitch collections to center around any pitch-class, we can create twelve unique pitch centers – called keys – and even more if we include enharmonic equivalents. Because writing accidentals for many of these keys would be clunky and difficult to read, we use a system of key signatures to give the performer a simple way of knowing which pitches in the key are raised and lowered.
The structure of key signatures relates to the very nature of diatonic harmony. This music is built around perfect intervals, and as we discussed in our topic on scales, the one non-perfect 4th/5th in the major scale is the one that creates the tension necessary to accentuate the tonic and therefore creates our tonal system. Because of this non-perfect interval, key signatures follow a very simple pattern that can be reversed whether you are raising or lowering pitches.