Masks: For all hybrid masks (masks that deliver air through mouth piece and nasal pillow). These liners are 1/4" thick. This Gel Liner seals the mouth piece of the mask. The upper flap of the gel Liner can be used to keep the nasal pillow in place inside the nostril during sleep. Sizes: Comes in one size that will fit all sizes of hybrid masks. Padding: Hybrid Gel Liner is 1/4" thick and is designed with a specific padding that is facing the mask which prevents the gel liner from moving during sleep by locking the liner inside the mask edges.