These are the posterior lateral pads for the JAY Encompass backrest, ordered by side and by size. They mount behind the shoulder blade line and hold the trunk from behind, which is what you want when a client tends to rotate or lean and you need to block that without crowding the front of the chest.
Because they are ordered left and right separately, you can set an asymmetric arrangement: one lateral higher or deeper than the other to answer a scoliosis or a habitual lean. That flexibility is the reason the Encompass is chosen for clients whose trunk shape is not symmetrical.
A personalised option is available where the standard pad shapes do not give the contact you need.
