Lachesis is the middle of the three Moirai (Fates), with Clotho and Atropos. She measures the thread of life spun on Clotho's spindle, apportioning each lifespan; her Roman counterpart is Decima. Myths place the sisters at a child's birth to fix destiny within three days.
Adapted from Wikipedia, "Lachesis" · CC BY-SA 4.0
I.Measurer of the thread
Clothed in white in standard iconography, Lachesis draws out and measures the thread Clotho spins and Atropos will cut. She determines how long each mortal lives and, in some texts, chooses the person's destiny during the measurement. With her sisters she appears within three days of a birth to decide the child's fate.
II.Parentage and Plato
Hesiod's Theogony first makes the Moirai daughters of Nyx, then later daughters of Zeus and Themis. Plato's Republic book 10 presents Lachesis as daughter of Necessity: she instructs souls about to choose their next life, assigns lots, and displays the human and animal lives from which they may choose. Cicero's Parcae are children of Erebus and Night; Renaissance mythographers (Boccaccio and others) continue to dispute the Fates' origin while keeping Lachesis as the measuring middle sister.