From the Halfmoon website:
Our Chip Foam Block with Cover is one of our most versatile props. The interior foam insert is dense enough to offer solid support, yet soft enough to provide padding for knees, elbows, or hips. The cover is removable and washable to ensure a long and durable life for your chip foam block. Whether seated, standing, reclined, inverted or twisted, this block can bring comfort, depth, lift, or support to any posture.