The bones of the shell are covered with horny scutes. The division between adjacent scutes is called the seam. A seam often leaves an impression, termed a sulcus, on the underlying bones. We follow the terminology for the various scutes proposed by Zangerl (1969) but further systems of scute nomenclature have been proposed (Systems of scute nomenclature).

Along the anterior midline of the carapace is a single cervical scute (Carapacial scutes of turtles). This is followed posteriorly by a series of five vertebral scutes. Along each side and touching the vertebrals is a series of four pleurals. Outside the pleurals and extending along each side from the cervical are 12 marginal scutes (11 in Kinosternidae). In the alligator snapping turtle (Macroclemys) a series of small scutes, called supramarginals, appears between the posterior pleurals and the marginals.

The scutes of the plastron (Plastral scutes of turtles) are divided into pairs by a median longitudinal seam. Anteriorly there is a pair of gular scutes (except that in the family Kinosternidae a single gular is usual); in some families an intergular is also present. Paired humerals, pectorals, abdominals, femorals, and anals follow, respectively, and in the Cheloniidae an interanal is sometimes present. At the posterior edge of each axillary notch there may be an axillary scute, and at the front edge of each inguinal notch there may be an inguinal scute. Inframarginals, a series of small scutes lying between the carapacial marginals and the sides of the adjacent plastral scutes, are present in the families Cheloniidae, Chelydridae, Dermatemydidae, and Platysternidae.

Included in the description of each hard-shelled species is a plastral formula that denotes the relationship of the length of each plastral scute, as measured along its midseam, to those of the other scutes on the plastron.

Turtles of the genera Pelusios, Emys, Emydoidea, Terrapene, Cuora, Cyclemys, Pyxidea, Notochelys, Pyxis, and Testudo have a transverse hinge, more or less developed on the plastron, and in most species of Kinosternon a pair of hinges borders the abdominals. These hinges allow the plastron to be folded up to enclose the head and limbs if it is large enough.

Turtles of the families Carettochelyidae, Dermochelyidae, and Trionychidae have lost the horny covering of scutes, and the bony material in their shells is much reduced. Instead, they have a tough, leathery skin. These turtles are often referred to as leatherbacks or softshells.