An adjournment pauses an unfinished game for resumption later: the player to move writes a sealed move, it goes into an envelope, and play resumes from that position another day.
When a classical game ran past the session, the player on move did not play their move on the board. They wrote it on their scoresheet, sealed it in an envelope along with the position and clock times, and handed it to the arbiter. At resumption the arbiter opened the envelope and made the sealed move, and the clocks restarted.
For most of the twentieth century this was simply how serious chess worked, and adjournment analysis became an art: World Championship matches of the Karpov and Kasparov era famously employed teams of seconds who worked through the night on adjourned positions.
Computers ended it. Once anyone could point an engine at the adjourned position, overnight analysis stopped measuring skill, and the practice died out at top level during the 1990s. Faster time controls that finish games in one session replaced it, which is why you will meet adjournments in chess books but almost never at a board.
At adjournment the player on move wrote their next move on the scoresheet instead of playing it, sealed it in an envelope with the position and clock times, and gave it to the arbiter, who played it at resumption.
Chess engines. Once anyone could analyze an adjourned position with an engine overnight, resumption stopped measuring skill, so faster time controls that finish games in one session took over.
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