I play brass (trumpet) and was always warned on assuming the tuning was accurate; even with the perfect player using the perfect compensation on the relevant va

I play brass (trumpet) and was always warned on assuming the tuning was accurate; even with the perfect player using the perfect compensation on the relevant valves the lower notes are definitely iffy (they tend towards flat and the compensation slides can't push them sharp). That perfect player is improbable though. On the lower notes I can bend them by in the order of a quarter tone without touching the valves. If I pick up the instrument on a cold day without first warming it, not just me, up then it will be probably a quarter tone or so flat for the first few minutes. If you leave it on your chair and come back to find someone's turned the heater on while you were away (happened to me before), it can easily be a semitone sharp, IF it's cool enough to hold in the first place! I love my trumpet but it's one of the last instruments I'd use to illustrate really precise tuning.