If you open an accordion, you will find that for every button there are two reeds, one which plays when in the pull direction of the bellows and one in the push. Sometimes these reeds are tuned to different notes/pitches for each bellows direction (diatonic accordions), sometimes to the same note/pitch (chromatic accordion).
A question I have always had and which I have explored in the past is whether it is physically possible to make a single reed that plays the same note/pitch in both bellows direction. (thus saving one extra reed for chromatic accordions)
From my somewhat brief research in the past it seems it is not physically possible to make such a reed in theory, though I was wondering if anyone has tried to do this in practice?