The period in a game between the opening and endgame.(noun)
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Use "middlegame" in a sentence
"The middlegame, which is typically the next 20 to 40 moves, puts the structured pieces into play."
"Fifteen years and two recessions into the commercial Internet, it's clear that our industry has moved into a new competitive phase - a "middlegame" in the battle to dominate the Internet economy."
"The first part of the book, covering problems in the opening and middlegame was published by Quality Chess as Questions of Modern Chess Theory."