Characters can only capture enemies that have a smaller Build compared to them, though mounted units are exempt from this rule except while dismounted. When a battle is initiated with the intention of capture, the character's Strength, Magic, Skill, Speed and Defense are all halved, so it is advised to weaken the enemy prior to a capture attempt, unless the character is powerful enough to inflict sufficient damage regardless of stat reductions.
Once an enemy is successfully captured, the character can trade with their hostage to take anything in their inventory. They may also release the hostage, although this counts as killing them gameplay-wise (whereas just holding onto them until the chapter's end counts as if they have never been fought). Ostensibly, releasing a captured unit counts as letting them escape the battlefield with their lives intact, in relation to the story.
Capture is an extremely important element in Thracia 776, especially since the game begins with no gold and selling stolen weapons is almost required to earn funds, unless the player exploits the arenas available.
Mounted or airborne units cannot be captured normally, unless they have been put to Sleep, in which case they are treated as though they have dismounted. Characters with a Build of twenty cannot be captured, though. Interestingly, unique enemies and bosses usually say different lines if they are captured and released, instead of merely defeated in combat.
Enemies may also attempt to capture the player's units, so keeping non-combative units, such as Priests, away from the front line is strongly recommended at all times. Any unit that is unarmed will always be captured in an attempt, as they have no means to defend themselves. It should be noted that if a unit is captured, the enemy is severely weakened, making them very easy to defeat and get your unit back; therefore, sometimes putting an unarmed unit in the front lines can be useful for defeating more difficult enemies.
In Fire Emblem: Radiant Dawn, a very similar mechanic exists, though it only appears in the Prologue of Part II; in this chapter, the opposing Begnion Dracoknights will capture Princess Leanne if she is within their range.