A piece's meter over time: the signatures in force, in beat order.
A meter change starts a new bar, so a span that does not divide evenly into
its own bars ends with a short bar rather than spilling across the change.
The first entry also governs everything before it, which is what reads a
pickup written at negative beats in the signature the piece opens in.
Beat 0 is the first downbeat here as everywhere else: the opening signature
lays its bar lines from beat 0 whatever beat the first entry is written at, so
a map opening at the pickup it covers numbers its bars exactly as the bare
signature does. Every later entry starts a bar at the beat it takes effect.
A piece's meter over time: the signatures in force, in beat order.
A meter change starts a new bar, so a span that does not divide evenly into its own bars ends with a short bar rather than spilling across the change. The first entry also governs everything before it, which is what reads a pickup written at negative beats in the signature the piece opens in.
Beat 0 is the first downbeat here as everywhere else: the opening signature lays its bar lines from beat 0 whatever beat the first entry is written at, so a map opening at the pickup it covers numbers its bars exactly as the bare signature does. Every later entry starts a bar at the beat it takes effect.