A crooked tune is a musical piece, generally in the American, Canadian, or Irish tradition, which deviates for the standard number of beats for that style of tune ( reel, hornpipe, polka). That is, the tune may add or drop notes, disrupting the usual rhythm. [1]
Banjo player Tony Trischka described crooked tunes as: Things aren't all foursquare. They're quirky around the edges. Just the way players of yore felt it. Instinctively correct rather than technically correct. [2]