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You'll want to make sure that the instrument you are tuning to is in tune before tuning to it. Some instruments, like the piano, stay in tune for long periods of time and are excellent tools for tuning your bass.  The E note on a keyboard can be found by locating the white key immediately after a series of two black keys in a row. Any key immediately after a series of two black keys is E. Other instruments that have recently been tuned, like a guitar or trumpet, can also be used to tune the E string of your bass. When you play your reference E note on an instrument, clear your head and try to hold onto the E note's pitch as clearly as possible. This is the first note you'll be tuning to.
Play the E note on an instrument.