An Almain rivet is a type of flexible plate armour created in Germany in about 1500.
It was designed to be manufactured easily whilst still affording considerable protection to the wearer.
[1] Almain rivets were generally of fairly low quality, but they were cheap: a royal proclamation issued by Henry VIII in 1542 designated them at 7s 6d, which equated to one sixth of the cost of a suit of demi-lance armor.
[2] Almain rivets were frequently purchased en masse as munitions-grade armour to equip royal armies or personal retinues.
Based on the term almain-rivet, the word rivet itself acquired a meaning of "armour", attested (rarely) during the mid-16th century.