As a guitar player, I have the opening low E note from 'Born On The Bayou' by Creedence Clearwater Revival burned into my brain - so, if I ever need to tune a guitar without a tuner, I use my brain memory to get the first note, and then move on from there . . . and I usually come pretty darn close to 440.
On the mandolin, I have similar luck with remembering Dave Grisman's version of 'Nine Pound Hammer' to use as a base line for mandolin tuning.
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