A Full helmet is armour worn in the headwear slot. Full helmets are stronger than medium helmets, providing a higher defence bonus, and are used primarily by warriors. The helmets, as with all melee oriented gear, give good defence against range attacks, but are weak against magic.
Full helms are heavier in comparison to medium helmets, some full helmets and their trimmed parts do have plumes. Trimmed and Gold-Trimmed armours have grey and gold plumes respectively, and the god helmets have plume colours representing the god to which they are aligned. Full helms while providing more defence than medium helms, but provides the same health bonuses.
On 1 July 2008, when Jagex released RuneScape High Definition, all full helmets except the Black full helmet were mistakenly rendered with red plumes, rather than the normal blue plumes. A Rune full helm looked identical to a Zamorak full helm. This bug was quickly fixed by Jagex.
A recent update has made it so that the bottom part of a full helmet moves as the player talks. Considering there is no joint, this looks very odd.
The helmets in Dungeoneering are referred to as full helms, even though there are no medium helmets in that skill.
Steel or Rune full helms can be painted with 48 Crafting - this makes them "heraldic"; their colour will reflect the colour of your ancestors, and it will have the appearance of an unupdated full helm, with the rims as a face, but they can be seen through. The joints also move when talking. Note: When doing this, they become untradeable, and cannot be reverted - the only reason to do it is for the aesthetic purpose. Also, they retain their Armour+Life Point bonus.