Around these parts, storm windows are typically 1 1/8″ thick. In these circumstances, double biscuits can be used as simple joinery. But sometimes we see thinner storm windows – ones that are only 3/4″ thick. Under this scenario, the wood isn’t really thick enough to install a double biscuit, and a single biscuit may not provide enough strength. So, alternative joinery may be in order. For a simple solution, drill a 3/8″ hole through the stile and into the rail and glue in corresponding dowels. Two dowels can be used for the upper and lower rails but you’ll likely be limited to a single dowel for the center mullion. Blind dowels can also be used.