A piece of mail armor hung from the lower edge of a helmet to protect the neck and shoulders.
From Old French 'avantaille,' formed from 'avant' (before/forward) and 'taille' (cut or piece). The mail extension evolved as helmets became more sophisticated in the 13th-14th centuries to protect vulnerable areas.
Knights wore aventails because they discovered through hard experience that your unprotected neck is basically an 'off-switch' in battle—so they added these draped mail extensions that looked like metal hair!
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