Two symbols turn up where a time signature should be, and both catch beginners out.
A large C = 4/4. Four quarter-note beats a bar, exactly the same as writing "4"
over "4". You'll see it constantly — Mozart's editions of K.216 use it.
A C with a vertical slash (¢) = cut time, also called alla breve or 2/2.
Two half-note beats a bar — the same total length as 4/4, but felt "in two", with a
quicker lilt.
The C is not a letter
It looks like a C, and people say "common time", but it was never the letter C or short for
the word "common". It's a broken circle, left over from medieval notation: a full circle
meant triple time, a broken circle — which happens to look like a C — meant duple. The slash
through it just halves the beat value again.
84 bpm
Common time — the broken-circle symbol, meaning 4/4. Count it in four: '1 2 3 4'.
100 bpm
Cut time — the same four quarter notes, but now they're two beats, not four. Each half-note beat holds two of them. Count '1 & 2 &' — much brisker on its feet.
Same notes, counted two ways
80 bpm
In common time you'd count this in four. In cut time, exactly the same rhythm on the page, you'd count it in two — the half note is one beat, not two. The notes don't change; the counting frame does.
Metre mark, or the note C?
72 bpm
The C right after the clef, in the time-signature slot, is the metre mark. The notes on the staff are pitches. Position tells you which is which — reveal to check the names.
The usual mistake
Believing the C stands for "common" (a nice mnemonic, but wrong, and it trips you up on the
slashed one). Or pausing mid-read to wonder whether a C is a note — its slot, right after the
clef and key, tells you it's the metre.