Abstract: This invention relates to an accessory article for those liquid filled bed mattresses that are commonly marketed as "Waterbeds" and/or "Water Beds". Installation to said type beds provides a rigid "auxiliary mattress corner" to which mattress cover and fitted sheet may be attached to remain securely and snugly fitted to said mattresses in normal daily usage. This invention may also be attached to conventionally constructed mattress, that is, fiber/spring and/or polymerics for the same purpose and result as with said type liquid filled mattresses.