Cut time sign (alla breve)

Feel two big beats per bar, each one a half note.

A bar of cut time holds the same notes as a bar of 4/4: four quarter notes, or two half notes. The difference is the pulse. The conductor beats two, and players feel the music in two long strides rather than four short steps. Counting a fast march in four makes it sound heavy and fussy; counting it in two lets it move.

The sign sits right after the clef and key signature, where the numbers 2/2 would otherwise go, and it means exactly the same as 2/2. It does not set a speed on its own; a tempo word or metronome mark does that. You meet it most in marches, show tunes, many hymns and quick movements of classical works.

Why is it a C with a line through it?

It is a medieval circle sign, broken and then cut.

In mensural notation, the system Europe used from about the 14th to the 16th century, a full circle meant “perfect” time, with the beat divided in three, and an open half circle meant “imperfect” time, divided in two. A stroke through the sign called for diminution: the notes went twice as fast. The half circle survived as the sign for 4/4, the struck half circle as the sign for 2/2. “Alla breve” is Italian for “at the breve”, the beat falling on a longer note value than usual.