Prone

Characters may sit, kneel, or fall prone as a no-move action. Getting up from sitting or kneeling is considered a half-move action, so a character can stand and still fire a missile or make an attack. Standing up from a prone position is treated as a full-move action, so the character can do nothing else in that round except rise.
Prone characters present very small missile targets and gain an AC bonus of –2 versus ranged attacks. However, they are very vulnerable to anyone close enough to threaten them. Melee attacks against prone characters gain a +4 bonus to hit. (Table 51 of the Player's Handbook)
Prone characters can only use crossbows or size S weapons while they're on the ground. A prone character firing a crossbow attacks at one-half the normal rate of fire and makes any melee attacks with a –4 penalty to hit.
Source: Special Combat Conditions - Combat & Tactics
Category:Status Effects