When looking at a muscle, you can often see that it appears to have lines running within it. These lines are composed of muscle fibers and the direction that these fibers run in relation to the midline of the body has been used to provide partial names to many different muscles.

If the fibers of the muscle are running with or parallel to the midline of the body, the term rectus is often used to describe that muscle. Rectus is of Latin origin and literally means "straight." Some examples of muscles that have the term rectus in their name include the rectus femoris and rectus abdominis.

If the fibers of the muscle run at an angle to the midline of the body, they are said to run obliquely. The term oblique is also of Latin origin. Some examples of muscles that have the term oblique associated with their name include the internal and external oblique muscles of the thorax.