All units listed below benefit from Thumb Ring. The Longbowman (denoted with a *) does profit from the accuracy increase Thumb Rings grants, but the Britons do not get Thumb Ring. Hence, this is only relevant in games with Full Tech Tree checked.
"The typical archer of the Middle Ages drew the bowstring back to his chest to fire. English longbowmen had the ability to draw back to their cheek, increasing the power and distance of their shots. Archers from Asia developed a thumb ring that made it possible to draw back to their ear or beyond. When this improvement was added to their already very powerful composite bows, their weapons proved quite effective."