It is much larger than the extensor pollicis brevis, the origin of which it partly covers and acts to stretch the thumb together with this muscle.

[2] Passing through the third tendon compartment,[1] lying in a narrow, oblique groove on the back of the lower end of the radius,[3] it crosses the wrist close to the dorsal midline before turning towards the thumb using Lister's tubercle on the distal end of the radius as a pulley.

In the metacarpal region, beyond the synovial sheath, the tendon is supplied directly from the radial artery.

[2] Tenosynovitis, inflammatory irritation of the synovial sheath, is relatively common in the third compartment after repetitive activities such as drum playing.

[5] This article incorporates text in the public domain from page 455 of the 20th edition of Gray's Anatomy (1918)