Metronome Synchronization

Putting them on the board allows them to couple, so vibrations from each can interfere and the most efficient energy transfer is when they are in phase. I’ve read that this trick was exploited by sellers of pendulum clocks, back in the day — by hanging the clocks on one wall, they would similarly tend to synchronize. This would give the customer the impression that all of the products were high quality clocks when that was not the case, though you could not tell from the pricetag — the poor clocks were being driven by a few good ones.

(Leroy Anderson was the composer of The Syncopated Clock)