Pegs have been used for a long time in the animation process, mainly for registration purposes. Peg holes at the bottom (or top) of animation paper are used to keep all of the sheets even and at the same registration.
There are three peg holes in regular animation paper. The centre one is circular and the left and right ones are rectangular and situated at 4 inches from the centre one.
To keep the drawings together, traditional animations use a peg bar. This bar has three pins shaped exactly like the peg holes. Peg bars are normally found at the bottom and top of animation discs.
Before digital compositing, the peg bars were also used to move layers under the camera stand. All layer translations were calculated based on the distance the pegs were moved frame by frame. They were the equivalent of the digital trajectories. Harmony makes use of these concepts to create animation and camera moves.