Space saving in this sense of the word means furniture. Modern living and their homes are compact and it's essential to find creative solutions.
For the past 40 years futon furniture has filled the need for space saving furniture. Because its a bed as well as a sofa. There are various wood or metal designs in the category and all have a separate mattress and frame.
Both these types of futon furniture have easy to use mechanisms to convert the frames. Some are able to convert from the front of the frame and other from the back, This ease of use is critical to being a space saving design.
Also especially relevant is the footprint of a particular frame. There are two and three seat double designs that are the same size when out as a bed. Due to the design of the 2 seat double that folds twice across its width there's a smaller footprint. As a result of the 3 seat double folding along the length it has a bigger footprint.