Trim boards, also called molding pieces, gracefully cover joints between different surfaces and materials. The most common applications are casings around windows and doors and base moldings at the bottom of a wall.

Crown molding is sometimes used to dress up the joint where wall meets ceiling, and various decorative trims, including chair rail and picture molding, are used to add interest to a wall or ceiling. If you enjoy installing or repairing a fireplace mantle, staircase parts, wainscoting or paneling, you will find a wide variety of trims made to suit those situations.

Trim pieces are identified by their profiles— how they look when viewed at their ends. You can find profile charts at home centers and from online suppliers to help you choose the right board for your purpose.

Wood trim is usually clear, meaning it has no knots, which (unless you are installing knotty pine paneling) would mar the appearance.

Though moldings made of other materials are available (see “Interior Trim Materials”), wood remains the most common material for trim boards. At a home center or lumberyard, you will find at least some of these options:

• Hardwoods make the best trim. They resist warping and cracking and can be stained to a beautiful finish. It is usually best to apply the stain before installing the piece, and then to touch up the cut edges and apply the final finish.

• Pine has long been the most common trim material. It resists cracking and remains stable. It usually has wide grain lines, meaning that it will have an informal appearance if stained. That’s why it’s usually painted. Pine is soft and so may be easily dented.

• Poplar is a bit more expensive than pine, but it is harder and more resistant to denting. It has a closer grain, so you may be able to stain it satisfactorily. Still, poplar is usually painted.

• Paint-grade trim is definitely meant to be painted rather than stained and is less expensive than clear pine or hardwood. Some types come primed, which is a big plus—you can probably get away with applying only one coat of paint rather than the two or three usually needed for bare wood. Paint-grade pieces are made of various materials.

• Finger-jointed boards are manufactured by joining together short pieces. The finger joint will be unattractive if stained, so these boards are usually painted. Some finger-jointed boards are better than others. Feel the joints to be sure they are perfectly smooth; if not, the joints may be visible even after painting.

When working on an older home, you may need to match a trim profile that is no longer sold. In this case you have three basic options:

1) Use one or more pieces of off-the-shelf trim that are similar to the old trim. Sometimes, combining two narrow pieces can mimic the look of one wider piece. This may be close enough that the difference is unnoticeable.

2) Have a lumberyard custom-mill pieces for you. There will be an initial charge for setting up the “knives” used for milling the profile. Other than that, the cost per lineal foot will not be too bad. Of course, that means that custom milling is expensive if you only need a few pieces but becomes more reasonable the more boards you purchase.

3) Start with clear lumber and mill it yourself. Use a router with router bits that match or come close to the profile you are after.

If your home has an Arts and Crafts style, you may find that much of the trim is square cut. In that case, you might be able to use clear 1-by boards (which after milling are 3/4 inch thick), or ask a lumberyard to mill boards that are a bit thinner or thicker.