the adjustable thin leg at the bottom of a cello or double bass.

Also called tail-spike [teyl-spahyk] /ˈteɪlˌspaɪk/.
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British Dictionary definitions for end pin

end pin


music the adjustable metal spike attached to the bottom of a cello, double bass, etc, that supports it while it is being played
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012