Jackknife sofa is a couch that can be pulled out to be converted into a bed. It comes in widths between 36"to 84". It was once a very popular choice, as it is the least space consuming bed and very portable.
It looks like an upholstered bench in the day, and at night, it can be laid flat with the seat and seat back lying side by side to create a bed. It usually uses a foam mattress, which helps prevent frames from poking on your back. But because they are less comfortable, some RVers will place another mattress on top for added comfort.