"Overblowing" or establishing an octave hole makes woodwind devices sound exactly one octave higher than the note would sound without overblowing except for the clarinet, as I will show below. Using the same fingering one time with and one without an overblowing hole makes the device sound in alternative hights; during this contents you speak of lower and upper check in. So you truly need seven tone holes and an overblowing hole. Learning how to play higher registers on woodwinds like a recorder is therefore rather simple, and it makes things easy for the tool maker: The tone holes and their distances for the upper octave are precisely the same as for the lower octave. Now overblowing works on clarinets, too, however the effect is alternative: The clarinet overblows not to the eighth tone on thescale an octave which is exactly double the frequency but to the twelfth tone. The Italian word for this is duodecime,and so we call the overblowing key the duodecime key. A amateur must learn this and get used to it. Furthermore this has implications on the construction of the tool: First it needs more tone holes than octavating instruments, as a result of if you are looking to play scales up, note 9, 10 and 11 need their very own tone hole. This means there must be more tone holes than we have palms. Then the tone hole positions for the lower scale may be alternative from that of the higher scale,especially their diameter. Since here is nearly not feasible, the instrumentmaker must find a compromise.