Beneath the obvious fabric covering any sofa or chair, three hidden elements exist; the frame, springs and padding. It is the combination of all four that make the piece as comfortable and durable as it is (or isn't).

Frame
Springs
Padding
Fabric

A kiln-dried hardwood frame is a must in ensuring durability in your piece of upholstery. Oak, maple and ash are woods most commonly used. A good frame is joined using dowels as well as corner blocks glued and screwed together. The legs should be an integral part of the frame and center legs should be used for additional support.

Springs

There are two primary spring systems used in constructing upholstered furniture; eight-way hand-tied and sinuous. Eight-way hand-tied springs are used in the base of better quality pieces and are often considered a sign of quality. These three dimensional coils are attached to webbing on the bottom of the sofa or chair and tied with twine at the top to each of the eight adjacent coils to prevent them from shifting. This system gives the product an even comfort level and has the advantage of never "bottoming-out" no matter how big the person is that sits on it.
Sinuous springs are two dimensional "S" shaped wires that are fastened to the top of the front rail and run from the front to the back of the piece every few inches apart. This spring system offers a somewhat softer seat and can be used in pieces that sit lower to the ground as in many contemporary designs. While less costly to produce, it is not necessarily the sign of an inferior product. The quality and quantity of springs utilized is more an issue than which system is selected.

Padding

The way a frame is padded will affect not only its comfort, but also its durability. Prior to applying the fabric, it is important that the frame be padded so that at no point will the fabric be touching wood. This will prevent untimely fabric wear and also add a subtle softness to the piece. Cotton and polydacron are the most popular frame padding materials. Seat cushions are filled with one of the following combinations of materials:

High density polyurethane foam wrapped in Dacron is the most common (and least expensive) seat core. The density of foam and quantity of Dacron will determine the softness of the seat.

Spring-Down cushions consist of innerspring coils surrounded by foam and wrapped in Dacron. This whole unit is then inserted into a muslin bag of channeled blendown. The springs give the unit support, while the blendown gives a luxurious surface feel.

Blendown cushions offer the softest seat, utilizing a combination of down and feathers sometimes wrapped around a foam core and then encased in a muslin bag. This cushion does not retain its shape without an occasional fluffing. Back cushions are primarily filled with Dacron fiber. Encasing the filling in a bag will help the cushions keep their proper shape. Occasionally, in better quality pieces, blendown is substituted for a more luxurious feel.

Fabric

When choosing fabrics for upholstered furniture, keep in mind that tightly woven fabrics tend to wear the best. Durability is determined by the number of threads per square inch rather than the thickness of the yarn used. Generally, fabrics that have their pattern woven in will wear better than printed fabrics.

There are many types of fiber. used in the manufacturing of upholstery fabrics. They each have unique attributes that must be considered in making a selection based on the planned usage of the piece. In addition to their look and feel, factors to consider are wearability, cleanability and sensitivity to direct sunlight.

Natural Fibers

Prized for their soft " hand" and versatility, the natural fibers remain popular as coverings for upholstered furniture.

Cotton

Cotton takes color well, soft and pliable, blends well with other fibers, durable with unlimited styling potential. However, continuous exposure to direct sunlight will cause the cotton fiber. to disintegrate. In damp climates, cotton may tend to mildew. It is somewhat less stain resistant than synthetics.

Flax/Linen

With a strong, cool, crisp " hand", linen is one of the most durable fibers available. It has a tendency to resist color and is often found in natural coloration's as a result. Linen reflects heat somewhat better than cotton, but will disintegrate in intense sunlight. It is at the higher end of fabric cost.

Wool

Naturally springy and resilient, wool is extremely durable. It tends to take color softly and has good resistance to abrasion. Wool has natural stain resistance, but should be mothproofed before using.

Silk

Without question the most beautiful, yet fragile of all upholstery fabrics. Soft and lustrous, strong light will tend to discolor and disintegrate the fiber. Difficult to clean, the fibers will tend to mildew in damp climates.

Rayon/Acetate

Made of processed cellulose, can be woven to emulate silk or linen. Its low cost and blendability with other more expensive fibers make it a popular choice. The fiber. is reasonably colorfast and abrasion resistant, but will tend to rot under long exposure to direct sunlight.

Synthetic Fibers

These petroleum-based fibers have been developed over the last sixty years as an alternative to natural fibers and are often blended with them in an effort to achieve the best properties of each.

Polyester: a polymer that takes vibrant colors well. Polyester is strong and cleanable and stands up well under direct sunlight. Flame and abrasion resistant, it is often blended with natural fibers to soften its feel. Recent developments have led to a finely extruded polyester fiber. that resembles the look and feel of silk.

Olefin: known by many people by the trade name, Herculon. Strong, with natural stain resistant properties, olefin is a bulky fiber. with a coarse "hand" that does not hold up well to direct sunlight. Often it is used to create heavy textured causal fabrics. These fabrics when loosely woven require a latex backing for added strength.

Nylon: The strongest and most dirt resistant fiber., nylon is used often in commercial applications where it will take a beating. Until recently, nylon typically had a high luster, but now it is often available with a delustered wool look and feel. Its only drawback is its sensitivity to sunlight.

Note:
While a few fibers have flame retardant properties, most others must be treated with fire retardant chemicals prior to being used commercially. Some states are now requiring this treatment on residential upholstered furniture as well.