Normal Front

On a normal front, the upper arm length matches the length of the shoulder blade. When the dog moves its front legs, the lower end of the upper arm (the elbow) swings below the level of the chest.

Terrier Front

On a terrier front, the upper arm length is short so that when it swings through its arc moving the front legs, the lower end of the upper arm (the elbow) swings at the level of the chest.