A pillow protector is a special cover, typically made with a dense weave, that fits over your pillow under the pillowcase. It serves to protect your pillow from dirt, bacteria, and allergens. More importantly, a high-quality pillow protector prevents feathers from leaking out of feather-filled pillows. Some pillow protectors are even designed with extra-dense thread-counts specifically to prevent feathers from leaking. We recommend using pillow protectors for all bed pillows, not just down and feather ones.