"Most of the helmets normally identified as used by legionaries have some form of plume or crest holder on the crown of the helmet but the precise method of attachment varies depending on period, location made and the type of helmet. The earliest forms known were probably similar to those depicted on early Greco-Etruscan pottery. The plumes and crests, based on the styles depicted on the pottery, appear to have used either feathers or naturally coloured horse-hair. If this style had continued in the Roman period then plumes or crests would normally have been a combination of white, black or red-brown in colour as horse-hair is difficult to dye using only natural vegetable dyes.
The earliest form of helmets appear to have only had centrally mounted plumes but in the early Imperial period late 1st BC to early 2nd century AD fittings have been found indicating that removable crest boxes might have been used. The evidence for crest boxes are mainly 'U' shaped crest holders which could be attached to fixing points in the centre of the crown and at the back of helmets. The lack of other non-metal remains indicates that crest boxes may have mainly been made out of wood so have rotted away over time. There also appear normally to have been either plume or possibly feather holders positioned on either side of the helmets. If feathers were used then these might have been goose feathers as sacred to Juno.
The purposes of these plumes or crests are thought to have been either for decoration, unit identification or as an indication of rank. Evidence from sculpture and monuments indicate that by the 2nd Century AD the crests were not used during combat and are mainly depicted only in use for parades or festivals. Vegetus is quoted as stating that centurions had a different form of crest and some sculptures of centurions show them with crests mounted transversely across their helmets, while representations of legionaries normally have the crest running from the brows towards the nape of the neck."
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https://www.romanobritain.org/8-military/mil_roman_soldier_helmet.php
