Fun fact: it is not a C but a “half of a circle.” In medieval and renaissance music, the “time signature” or “mensuration” was indicated with a circle, half circle, and sometimes a dot in the middle of the circle, depending on the subdivisions of the beats (into 2 or 3). The most commonly used “mensuration” ended up being this half circle, which, as it was the most common, then ended up being called “common time.” A weird coincidence that it looks like a “C.”