From Wikipedia - Reading time: 5 min

A chase is a heavy steel frame used to hold type in a letterpress.[1] Most of the space in the chase not occupied with type is filled with blocks of wood called furniture. The type and furniture are locked in place by quoins.[2] When a chase is locked up with type, furniture, and quoins, it is called a forme.
This brings the quoins at the top and right end of the chase and opposite the guides to which the sheets are fed.