Saxophones, oboes and generally generally most woodwind instruments I can think of behave like open pipes. That seems logical; when every key is closed, they actually are open pipes and, if I remember correctly, a pierced pipe behaves like a (slightly longer) open pipe.
The clarinet, however, behaves like a closed pipe. This has two major consequence I can think of: the partials of a clarinet songs consists of odd harmonics (at least in first approximation); a clarinet register key makes the instrument play a twelfth (thrice the fundamental) instead of an octave (twice the fundamental).
What is the physical explanation behind this phenomenon?