If you are talking about typical 18th century British/American looking glasses, the glass lays in the rebate formed by the moulding attached to the frame. it is held in place with a series of right-angle triangular shaped glue blocks roughly 2 inches long around the perimeter. One square surface is glued to the frame, the other faces the back. The hypotenuse rests on the glass securing it. The thin backboards are held in place by nails driven into the frame.