Most woodwinds will change octave by you climbing up the tube length, removing fingers. You then change your embouchure, open certain keys located at nodes to "vent" for some pitches fit the second octave, and you come to the third octave - built one one third of the sounding length,using a third or is multiple division (6). That is why D1 and D2 on the flute use most of its length - but D3 is a vented overtone of G1. If you walk your fingers up the clarinet, you cover about an octave and a fourth/fifth before you change gears, open the octave key to vent, and dive downwards to use the keys close to the bottom again. We say the clarinet "overblows a fifth" because its construction does not change *octaves* at the highest open hole in the instrument. You can find FAR better and more detailed information online.