The heart line is the uppermost of the major creases, running across the palm below the fingers, and the tradition reads it for affection, emotional life and, in the older texts, for the physical heart. Its length, depth, curvature, branching and breaks are all given meanings: a line curving up toward the fingers is read for warmth of expression and a straight line for reserve, chains for changeability, breaks for loss. Anatomically the major palmar creases are flexion creases, formed in the foetus by about the twelfth week of gestation at the sites where the skin folds when the hand closes, and their basic pattern is set before birth and remains stable through life. The tradition's claim that the lines change with the life is not supported; what changes is the fine secondary creasing, which is largely a matter of skin condition and use.
Aristotle's History of Animals already remarks on the palm's creases in the fourth century BCE, distinguishing the long-lived, whose palm shows one or two lines running right across, from the short-lived, whose lines do not extend the full width, a genuine ancient observation predating the developed reading tradition. That tradition owes much of its modern form to William John Warner, known as Cheiro (1866-1936), an Irish palmist whose bestselling Cheiro's Language of the Hand (1897) read the hands of clients including Mark Twain, who wrote that Cheiro had exposed his character to him with humiliating accuracy. A rarer variant worth noting is the Simian crease, where the heart line fuses with the head line into one transverse crease; it appears in a small share of the general population and more often among people with Down syndrome, though not diagnostic on its own, and palmistry has long read it as an unusually single-minded fusion of feeling and thought.