It used to be that only sofas or loveseats were convertible into beds. Styles were so unimaginative that the sofa-bed was instantly recognizable as such.

Today just about everthing in seating is instantly convertible into sleeping space. Chairs, sofas, settees, loverseats and chaises all convert readily into sleeping space for overnight guests. And the best part is that they're scarcely more expensive than the unconvertibles.

The practicality of this kind of dual-purpose furniture is obvious. We use our living rooms less than our kitchens or bedrooms. Why shouldn't they also serve as guest rooms? Especially since few of us have constant overnight guests, the double use of the parlor as guest room is quite justifiable.

In a living room I designed for a young couple in a city apartment, they had to use the parlor as a guest room. The overall space was small. The living room had a dining space at the end of its length, and there was only one small bedroom.

Without any other choice we set about making the guest quarters as comfortable as possible within the boundaries of the living room. I used a combination of a loveseat that opens into a double bed and a chaise that, in a pinch, could be used as extra sleeping space. Fortunately, there are manufacturers who combine these shapes in one line -- some of them can be used as sleeping sofas. I could have used two loveseats, one convertible and one conventional, but I liked the idea of a chaise, to make the furniture grouping that much lighter in look for such a small space.

Even better, the furniture could be rearranged easily because both pieces are on fat swivel casters. This means that the chaise, which is the same length as the loveseat, could be moved right up to it and the two used together, without bothering to open the loveseat as a double bed. And since the furniture had the same modular measurements, they could even be arranged in a closer, overlapping L-shape, to save more space, if the room had been smaller.

I covered the pieces in an enormous cottage check in beige and white, making two pillow covers of the fabric to fit standard-sized bed pillows. A corner table -- a 30-inch cube, 18-inches high, in a plastic laminate -- is the same sandy beige color as the background of the sofa. This completes the L-shape of the grouping and is a great spot for a television or a large flowering plant.

Instead of coffee tables, I used two large cubes of clear lucite. They give a feeling of greater space and allow the patterned rug to show through. Because they are on casters, they can be moved easily when the furniture is converted into a sleeping space. Walls painted the sme beige with white woodwork and shutters at the windows, plants and brass lamps and a lovely pale almost-white abstract painting on the wall, all combine to make the whole group fit easily into their second role as bedroom for the occasional guest.

This modular-plus-sleep furniture has gone a long way toward liberating our thinking about living room furniture. The modules are so versatile and flexible that they can move while we live in today's apartment, and move easily into a new and different-shaped space in our homes tomorrow. With our "here today, gone tomorrow" kind of overnight guests, they are the very answer to the seat-sleep problem.