Actually, even non-adjustable things can be calibrated; although the meaning would be somewhat different.
If the instrument is adjustable, then calibration involves getting the instrument to match reference standard, by changing the gain or offset or both.
In the case of something like a voltmeter, what is sometimes done is that the measurement remains the same, only the display is different. If the measurement is 1.0 V, but the calibrator says that it should be 1.06V, then anytime you get a measurement of 1.0V, you output 1.06V
Likewise, in the case of a ruler or protractor, the calibration would be the creation of a lookup table with the correct values, so that if you measure 60 degrees, you look up that value in the table and see what it really corresponds to, based on the calibrator.
TTFN