Mississippian period ceramic assemblages in the Nashville Basin region of Tennessee are traditionally viewed as being overwhelmingly shell tempered. Our petrographic analyses of 30 ceramic sherds from three Middle Cumberland sites have revealed, however, the presence of grog, or crushed potsherds, in shell-tempered pastes in over 40% of our specimens. In our study, serving vessels are often tempered with both shell and grog, with one bowl rim containing solely grog. Cooking vessels tend to be tempered with coarse shell and contain only incidental grog. Grog tempering alongside shell has been only occasionally noted elsewhere in the regional literature, but the lack of its widespread recognition may be due to the difficulty of identification without the assistance of a petrographic microscope. It is not clear whether the addition of small grog particles to a shell-tempered paste offers any immediate functional advantages. Other studies suggest that grog temper could improve the workability of the clay, may reduce thermal shock, and may enhance a vessel's resistance to mechanical stress. The strong correlation of fine grog and shell temper with bowls, however, may constitute a low-visibility horizon marker for an extensive swath of the Late Mississippian culture area.