When the brothers commit the taboo act of Human Transmutation to resurrect Trisha, it backfires and they face the consequences via the Law of Equivalent Exchange: Ed loses his left leg, while Al is dragged into the Gate of Truth. Ed sacrifices his right arm to save his brother's soul & bind it to a suit of armor. His missing limbs are replaced with "automail" prosthetics made by the town's automail mechanic whose granddaughter, Winry Rockbell, is the brothers' childhood friend.