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play by ear

ear, play
part of speech: phrase
definition 1: to play (a piece of music) just from the memory of the sound of it and not from reading the musical notation.
She never learned to read music, but if it was a song she knew, she sat down at the piano and played it by ear.
definition 2: to go forward with (some activity) without any previous planning or instructions, dealing with any problems just at the time they arise; improvise.
We've never encountered a problem like this before, so we'll just have to play it by ear.