The tail accounted for half of Deinonychus's body length and was used as a stablelizer or counter balance for an active, mobile, bipedal animal (Wilford, 1985). The tail was encased/ensheathed in tendons that stiffened it to keep it from dragging on the ground (Desmond, 1975). These tendons were able to reach 18 inches in length (Desmond, 1975) and were able to cover as many as twelve vertebrae at one time (Farlow et al, 1997). The tendons were actually bony extensions of the vertebrae (McGowan, 1991).

The bony rods in the dorsal (upper part) of the tail were elongated anterior zygapophyses, while the ventral ones (lower part) were anteriorly drawn out chevron bones (McGowan, 1991). These bony extensions are different from the ossified tendons seen in Ornithischians in that ossified tendons are caused by the connecting tissues of the vertebrae becoming filled with calcium and litterally are 'turned to bone'. The tail was most mobile anteriorly and immobile distally, which is the oposite from Ornithischians and their ossified tendons (Farlow et al, 1997). This means that the tail could move up and down, sideways and all-around-the-clock (Wilford, 1985).