4 : aerophones
42 : Non-free aerophones

421 : Edge-blown aerophones or flutes
421.1 : Flutes without duct
421.11 : End-blown flutes
421.111 : Individual end-blown flutes
421.111.1 : Open single end-blown flutes
421.111.11 : Without fingerholes
421.111.12 : With fingerholes.
421.111.2 : Stopped single end-blown flutes
421.111.21 : Without fingerholes
421.111.22 : With fingerholes
421.112 : Sets of end-blown flutes or panpipes
421.112.1 : Open panpipes
421.112.11 : Open (raft) panpipes
421.112.12 : Open bundle (pan-) pipes
421.112.2 : Stopped panpipes
421.112.21 : Stopped (raft) panpipes
421.112.22 : Stopped bundle (pan-) pipes
421.112.3 : Mixed open and stopped panpipes
421.112.21 : Mixed (raft) panpipes
421.112.22 : Mixed bundle (pan-) pipes
421.12 : Side-blown flutes
421.121 : (Single) side-blown flutes
421.121.1 : Open side-blown flutes
421.121.11 : Without fingerholes
421.121.12 : With fingerholes
421.121.2 : Partly stopped side-blown flutes
421.121.21 : Without fingerholes
421.121.22 : With fingerholes
421.121.3 : Stopped side-blown flutes
421.121.31 : Without fingerholes
421.121.311 : With fixed stopped lower end
421.121.312 : With adjustable stopped lower end
421.121.32 : With fingerholes
421.121.321 : With fixed stopped lower end
421.121.322 : With adjustable stopped lower end
421.122 : Sets of side-blown flutes
421.122.1 : Sets of open side-blown flutes
421.122.2 : Sets of partly stopped side-blown flutes
421.122.3 : Sets of stopped side-blown flutes
421.122.31 : With fixed stopped lower end
421.122.32 : With adjustable stopped lower end
421.122.33 : Mixed
421.122.4 : Mixed sets of stopped side-blown flutes
421.13 : Vessel flutes (without distinct beak) The body of the pipe is not tubular but vessel-shaped
421.2 : Flutes with duct or duct flutes
421.21 : Flutes with external duct
421.211 : (Single) flutes with external duct
421.211.1 : Open flutes with external duct
421.211.11 : Without fingerholes
421.211.12 : With fingerholes
421.211.2 : Partly stopped flutes with external duct
421.211.21 : Without fingerholes
421.211.22 : With fingerholes
421.211.3 : Stopped flutes with external duct
421.211.31 : Without fingerholes
421.211.32 : With fingerholes
421.212 : Sets of flutes with external duct
421.22 : Flutes with internal duct
421.221 : (Single) flutes with internal duct
421.221.1 : Open flutes with internal duct
421.221.11 : Without fingerholes
421.221.12 : With fingerholes
421.221.2 : Partly stopped flute with internal duct
421.221.3 : Stopped flutes with internal duct
421.221.31 : Without fingerholes
421.221.311 : With fixed stopped lower end
421.221.312 : With adjustable stopped lower end
421.221.4 : Vessel flutes with duct
421.221.41 : Without fingerholes
421.221.42 : With fingerholes
421.222 : Sets of flutes with internal duct
421.222.1 : Sets of open flutes with internal duct
421.222.11 : Without fingerholes
421.222.12 : With fingerholes
421.222.2 : Sets of partly stopped flutes with internal duct
421.222.3 : Sets of stopped flutes with internal duct
421.3 : Mixed sets of flutes

Source: https://en.wikipedia.org/wiki/Hornbostel%E2%80%93Sachs