When you sit on a sofa or upholstered chair, you know immediately whether it's comfortable. Have you ever wondered what makes it so?

It's the padding and covering that do the trick.

Way back before our time, upholstery consisted mainly of textiles, rushes or leather stretched over frames. Whichever, the result was so uncomfortable that people had to add loose cushions to ease the fanny fatigue. Nowadays, we make the frames, padding and covering comfortable by themselves, and the added cushions are mainly for looks, albeit extra comfort, too.

There are several paddings, stuffings and cushions on the upholstery market today that add comfort as well as beauty to furniture. One such comfortable seating would be a padding made from thin layers of resilient materials covered with fabric and secured to a frame. Until recently, long hair (usually horse hair) was the best and most costly padding, with kapok, moss and cotton used in the less-expensive upholstery. But because those ingredients cause allergies for many people, the great minds at the manufacturing plants discovered foam, and thick foam pads have now taken the place of stuffing in many seating pieces.

The pads are supported by plywood or webbing and have even become furniture by themselves. I'm sure you've seen one piece or several pieces of pads held together by buckles or straps, creating a chair, sofa or bed.

The old horse hair cushions made cleaning difficult, while today's cushions are often covered with zippered fabric that is removable for easy cleaning. In some cases, the foam padding is encased in a plastic laminate that can be wiped clean with a damp sponge. I don't recommend the plastic-wrapped foam because it is not very comfortable to sit on and makes a crinkling noise when you move around.

Here's what goes into sofa upholstery, from the inside out:

--The frame should be of strong hardwood such as oak, maple, birth or any leaf bearing tree, secured at the joints with metal or plastic.

--Webbing should be woven in a simple basket weave and tacked to the frame.

-- Springs are next, and the best are eight-way hand-tied, like the mattress on your bed. The next best is sinuous spring coil. The coil should be tied to the webbing and frame and spaced closely enough to prevent sagging.

--Burlap covers the springs to protect the padding. Muslin will be found only on better furniture. This material is used instead of burlap to cover the springs and protect the padding.

--Fabric is the icing on the cake. It should be of durable quality and aesthetically pleasing. It should be comfortable to the touch, with a degree of resilience.

--The cushions and pillows come three ways: loose, semi-attached and attached. They are all good. Down remains the most luxurious stuffing for the top cushions/pillows, but it's also the most expensive. Foam with a polyester wrap is the most popular type of cushion today. Several manufacturers of higher end furniture offer their cushions in degrees of firmness, as some people like to sink into a chair or sofa, while others prefer a greater amount of firmness and support.

The bottom line is: Take a seat: If it's comfortable to you, buy it.