Helms

Campaign Use: Helms are part of a warrior's basic protection. Failure to use proper headgear leaves a character open to called shots by opponents or to other disadvantages at the DM's option. All headgear except that which does not cover the ears must be removed when making rolls for listening.

The Complete Fighter's Handbook offers an optional rule for Hearing and Vision checks. All helms restrict Hearing and Vision checks to some degree. (A Vision or Hearing check is a 1d20 roll against a character's Intelligence or Wisdom, whichever is higher. These checks can be used when a character has a chance to hear or see something of importance. This check isn't used when characters listen at doors; that has its own check.)

Optionally,
great helms and closed-face helmets, when worn with plate armor, gain bonuses against dragon's breath and spells that affect the eyes (see the Equipment section in The Complete Fighter's Handbook).

Helms and Thieves: Thieves suffer automatic penalties to hearing if a helm is worn. The following descriptions include penalties to Hearing and Vision checks. If these rules are not used, the penalties for Vision checks should be ignored; for Hearing checks, substitute a -5% penalty to the thief's Hear Noise ability for each -1 Hearing check modifier (e.g., a -3 to a Hearing check is a -15% to the thief's Hear Noise roll).

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