These are triangular fabric pockets at the corners of the fitted sheet. You insert a plastic or wooden pole into the sleeve, which then tucks under the mattress. The weight of the water holds the pole and sheet firmly in place.
These look like traditional mattresses and use a foam border to hold the water. Most softside models are compatible with standard bed sheets (Twin, Full, Queen, King), making them easier to shop for. 2. Specialized Attachment Features buy waterbed sheets
Because waterbed mattresses move (the "wave action"), hardside sheets often include features to prevent them from slipping off: These are triangular fabric pockets at the corners