By the end of the 1600s, Europe was beginning to record its first criminal autopsies, describing, among other things, the lungs of deliberately smothered children -- which were noted to be fluid-filled and speckled with blood. These are the same signs that a forensic pathologist sees today at autopsy.

The French developed the concept of lividity, or livor mortis, from the French liviere, "to turn blue." This is the gradual deoxygenation and gravitational settling of the blood that begins as soon as lungs and heart cease their motions.

The color progression of lividity begins with the proverbial pallor of death, as blood begins to drain out of the upper surfaces of the body. Fifteen to twenty minutes after death, an experienced observer can see the first diffuse blotches as they appear on the underside of the body. The seepage also becomes visible in dependent areas such as earlobes and skin folds. Within an hour or two, the telltale discoloration becomes obvious to even the untrained eye, even in a person who is dark-skinned. The pink color of early livor gradually darkens to a dull, bruiselike red before progressing through shades of purple and blue as oxygen gradually disappears from the blood.

The lividity is not yet "fixed," or permanent. Press your thumb against an area of livor in the first hours after death, and it will blanch. Similarly, should you move the body during this period, the blood-settling patterns will shift, though perhaps not completely, for livor's fixation is gradual. A body dead in a kitchen chair at 5 p.m., then undressed and tucked into bed at 8 p.m. may retain the faintly blanched impressions of contact points between the body and unyielding surfaces such as the back of that chair or a tight waistband, thus allowing the trained observer to notice that something is amiss.

By ten hours past death, the color has become fully fixed. The body has now cooled to the point where the fatty lining of the blood vessels congeals, pinching shut the tiny capillaries near the body surface. The dark stain of blood seepage can no longer escape inward when pressed, nor will it resettle, even partially, when the body is shifted. Moving a body once livor has fully set leaves behind a stark and permanent imprint of its original position.

Oftentimes, the detail can reveal the very texture of the surface on which the victim expired -- be it the stippled inscription of a gritty path, the weave of a carpet, or the design of kitchen linoleum. Even after it has becomed fixed, the stain of lividity may continue to darken, reaching its maximum intensity around twelve hours postmortem. It will remain prominent until overwhelmed by the greenish "marbeling" of the skin's surface -- named for its resemblance to the veins in marble -- which is the beginning of decomposition.