What’s it called when つ becomes っ, like in 発見? And is it just me or is there a pattern for when it happens?
The linguistics concept is called gemination.
It basically happens from the difficulty that would arise from saying things like unvoiced mora like け immediately after things like く, つ, or ち (and probably others).
Any thorough description would probably be difficult to memorize entirely and still have exceptions though.
You get used to the situations over time.
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Also, the name for the っ itself is 促音
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