Why Does a Contrabassoon Fold Its Long Bore Twice?

The contrabassoon sounds an octave below the bassoon. Its register is determined not by how tall the casing appears, but by the effective length of the air column beyond the reed. If its conical bore were stretched into a straight line, the instrument would be impractical to hold, let alone finger across widely separated tone holes.

The solution is not to shorten that acoustic length, but to fold the bore back on itself twice, rather like a paper clip. The air still travels along a very long route, while the player handles an instrument that can stand in front of the body and be operated through a key system. Folding changes the geometry, not the length required for the low register. A bassoon already turns its bore back once; the contrabassoon adds another fold to reach an octave lower within a manageable height.

Listen beyond the biggest low notes. What makes the contrabassoon distinctive is the grain of its double-reed attack combined with a sustained tone that can draw the orchestra's bass into a deep, concentrated line. It can reinforce the foundation, but it can also give a melody weight, shadow or dry humour. The long tube has not vanished through folding; it has been fitted into a form that one body can control.

https://philharmonia.co.uk/resources/instruments/contrabassoon/
https://www.yamaha.com/en/musical_instrument_guide/bassoon/structure/structure002.html