The plural form referring to multiple things numbered 15, such as rugby teams with 15 players per side, or groups of 15 people.
From fifteen plus -s (plural). In rugby, 'fifteens' specifically refers to the standard 15-a-side game format, distinguishing it from 7-a-side (sevens).
In rugby culture, calling it 'fifteens' is how players distinguish the classic full-contact version from 'sevens' (the speedier, 7-per-side tournament format).
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