Cold frames have been used for centuries (and are still used) as a means to grow tender plants or 'harden off' seedlings for planting out in the garden. Generally frames are built in a series of bays (as in the picture to the left), the back wall about half as high again as the front, and each bay covered with glazing that can be slid back to expose the entire bed within the bay (again, as in the picture to the left), or just partway in order to ventilate the plants. Sometimes cold frames can be filled with manure to make a hotbed within them (when technically they cease to be 'cold' frames) on which to grow tender plants, or vegetables throughout winter. Generally, however, cold frames are used to 'harden off' seedlings and young plants that were initially cultivated in the hothouse or greenhouse for a few days before they are planted out in the garden. Historically the walls of cold frames have been made of brick.