I was in a piano shop yesterday and saw someone touching up an upright, checking the tuning by alternating M3's and m6's. In other words, he played C4 and E4 together,and then E4 and C5, and then C5 and E5, etc up and down the keyboard, going through the cycle of 5ths. (Within a given key, this would be checking on major 6ths. I'm aware that the term m6 seems odd--perhaps I should say that he was checking on M3's and #M5's or bM6th's?)

He seemed to be listening for, and getting, a similar color for both intervals. I remember seeing this many years ago, but I'd forgotten it. The resulting sound was good--it ensured that the thirds within each octave were good, of course, and that the overall sound was consonant when moving to the next octave. He finished by checking M10's, which also came out sounding good. Lovely chords in all of the keys.

I couldn't remember anyone mentioning this (old-fashioned?) check here. Does it encourage less stretch\a narrower octave--does getting a similar color in the M3's and M6's keep the M5's narrow? Or does it encourage other problems?

m6 is the correct notation for the interval from E3 to C4. If what the tuner was doing was comparing the beat rate of E3 - C4 with the beat rate of C4 - E4 then he was using the m6-M3 test for 8:4 octaves. But that is unlikely. 8:4 octaves produce enough stretch to make 5ths beatless and octaves busy. He was probably just listening to how things generally sounded.

I think that I was caught by surprise partly because this M3, m6 sequence meant that he was tuning to get the intervals within keys good (both within an octave and as the key spread to the next octave), instead of tuning to get intervals right without regard to key.

But what made a stronger impression was how the other intervals just seemed to fall into place--once he'd run the test through the cycle of fifths over all of the octaves, the M4's and M5's were good--it was as if getting the M3's and their inversion good in all of the keys automatically made everything else line up. Of course, I may have just come in late--he may have started by setting the intervals in other ways and already have pitched the octaves by listening to beats, and then was just cleaning up by listening this way. He seemed to be adjusting for the "color" of the M3's and m6th's, though.