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What is a band?
A shared name, a shared set of records, and people who may leave. The name often outlasts the lineup.
A band starts when a few players agree on a name and put it on a flyer. Then the name sticks. Members quit, get fired, die, or rejoin for a tour. The logo stays on shirts and radio playlists.
Led Zeppelin broke up; the records still circulate. Nirvana stopped; the quiet-loud dynamic still shows up in garage bands. That is the point: a band is not "a singer plus backing." It is temporary chemistry frozen onto vinyl, CD, or files.
On this site, "decisive" does not mean "best." It means later bands had to react. The Beatles made the album the main unit. Black Sabbath made heavy guitar the default language for metal. Those reactions fill the timeline.
Start with something you already like. Then ask: who else was around when this sound solidified?