The most important part of the Murphy bed is the frame itself. This is a specialized folding bed frame. The top portion of the frame is typically just slightly less wide than the space the bed is placed in. This portion of the frame has a hinged pivot on it. The rest of the frame then folds upward, pivoting on the top portion, to fold into the closet. The frame is ultimately what makes a Murphy bed work.
Murphy bed frames are outfitted with a folding mechanism that makes it easy to manage the weight of the bed as you fold it out or put it away. Usually, this consists of a pair of high tension springs that may be attached to the doors on the cabinet. Other Murphy beds have pistons that are pressurized to support the bed when you put it away or take it out. The upside of pistons is that most are equipped with locks that ensure the bed doesn't fall out accidentally when not in use.
To fold up, a Murphy bed needs a compartment to fold into. Usually, this is a piece of furniture that resembles a large storage cabinet or wardrobe. Ideally, the piece should be functional as well as storing the bed, such as having additional storage for clothing or linens. These compartments need to be strong enough that the bed can be anchored to them at the required points.
The mattress is the final important component of a Murphy bed. If you don't buy the mattress as part of a package with the Murphy bed mechanism, frame and cabinet, you need to buy one that will fit the rest of the device. Not only is the bed frame size important, but you must also consider the depth of the Murphy bed cabinet. A cabinet that is only 12 inches deep can obviously not accommodate a 16 inch deep mattress.