Most daybeds are twin-size mattresses, making them 38" deep. The seat depth of an average couch is only 20-25". Daybeds don't fold out to become beds; instead, you just remove whatever's on the daybed during the day so you can use it as a bed at night. Futons are sofas that have one large futon mattress. A futon sofa folds back flat to make a bed. A trundle bed is a bed that pulls out from underneath another bed to create a second sleeping space. When it's not in use, the trundle bed goes back underneath the main bed, so it's not in the way. A trundle bed can go under a normal bed or a daybed. A sofa bed is a couch that has a metal frame under its seating cushions. The metal frame pulls out from under the cushions, revealing a mattress to sleep on.