Beatmatching
The DJ craft of locking two tracks to exactly the same tempo and phase so the transition goes unnoticed.
The technique is usually credited to Francis Grasso in New York around 1970. On vinyl it goes like this: cue the next record in headphones, ride the pitch control until its beats sit still against the one playing, then nudge the platter forward or back to correct the phase. The Technics SL-1200MK2 of 1979, with quartz-locked speed and a ±8 percent pitch fader, was the trade's tool for thirty years.
Software now detects the tempo and lines up the beats with a sync button. This has sparked an eternal debate about whether that counts as cheating, conducted mostly by people who learned the hard way. Time-stretching also means the key can be held when the tempo changes.