By MATT MAURY

An interesting aspect of the English language is that, while descriptive words remain the same, the item being described can change in its physical characteristics. For instance, consumers are disappointed to find that the "pounder" size bag of candy now contains only 14 ounces of product.

And, for do-it-yourselfers in older homes, the changes in wood product sizes can be downright frustrating. A 2x4 isn't really that size any more, nor is 1/2-inch plywood really the full size it once was. So, when a small repair job is attempted, the lumber or plywood you buy today probably won't match what was originally put in the home. The evolution of wood product sizes has been affected by a combination of milling technology, engineering and conservation. For lumber and plywood, the courses have been different.

LUMBER

Back in the olden days, the early 20th century, the production of lumber was done by sawing the logs into full 1-inch and 2-inch slabs, which were cut into various widths. Much of the lumber was sold "green" and the stated thickness and width were accurate. A 2x4 was truly 2 inches thick and 4 inches wide. The lumber was rough, the carpenters got slivers, so the demand for "surfaced" lumber that was run through a planer increased. Of course, taking a little bit of wood off of each side meant that a 2x4 was no longer a full 2 inches by 4 inches.

Still, even after it was planed, the green lumber still was prone to warping, twisting and cupping as it dried unevenly. Thus, many mills dried lumber uniformly before surfacing it. Fact: as lumber dries, it shrinks. By the time the dried lumber was surfaced, the 2x4 netted out at 1-5/8 inches by 3-5/8 inches.

This was the industry standard until the mid-1960s when scientists determined that lumber dried to a 15 percent moisture content could be planed to 1-1/2 inch thickness and still have the same physical properties as the traditional lumber for each given species. This prompted the adoption of the American Software Lumber Standard PS 20-70 by the U.S. Department of Commerce in 1970, which has defined the sizes of lumber (as well as many other attributes - grades, physical properties, etc.) for the past 31 years. A nominal 2x4 nets out at 1-1/2 inches by 3-1/2 inches.

Up to here, we've used the 2x4 as an example, yet the changes in size go throughout the range of lumber thicknesses and widths. For homeowners trying to match a few pieces of solid wood paneling or siding for a repair project, the search can be both frustrating and expensive. Don't expect much help from the big chain building material stores. The best bet is in going to a local lumberyard that does custom milling (they can be found in the Yellow Pages). For those do-it-yourselfers that desire more knowledge about the various species of lumber, a good source is the Western Wood Products Association Online Technical Guide at www.wwpa.org.

PLYWOOD:

For the past 50 years or so, a standard building component in homes has been 1/2-inch by 4-foot by 8-foot structural plywood. There's also 1/4-inch to 1-1/8-inch thick plywood, but as with the lumber example, we'll stick to one common size. When you go to the store to buy a sheet of plywood, you might find that, while the bin is labeled as 1/2-inch CDX ("C" and "D" being the grades of the veneer in the product, while "X" is for exterior), all the panels in the bin are stamped 15/32-inch. Getting out a tape measure, you might find that some of the panels are really only 7/16-inch thick.

Unlike lumber, which found nominal sizes reduced in the drying and surfacing processes, plywood's nominal thicknesses have been reduced by technical adaptations. Prior to 1980, 1/2-inch plywood was a full 1/2-inch thick. In 1981, the APA (first the American Plywood Association, then the American Panel Association, now APA-The Engineered Wood Association) instituted PRP-108, a new set of technical performance standards for panel products. In non-technical terms these standards are span tables and deflection limits. Scientists at some of the major plywood producers determined that the new standards could be met with slightly thinner plywood and those companies began manufacturing "scant" plywood sizes. Because wood veneers are not totally uniform in thickness, there is a tolerance allowed in the manufacturing process of 1/32-inch. This can result in panels as thin as 7/16-inch or as thick as 1/2-inch.

While manufacturers are properly labeling the panels as nominally 15/32-inch thick, many retailers and consumers still call it 1/2-inch ply-board or 1/2-inch sheathing. For do-it-yourselfers that are trying to replace rotted or damaged plywood in an area where it abuts to other plywood, it is important to know that many panels over 1/2-inch thick have been manufactured to these "scant" sizes since the early 1980s. As with lumber products, the local lumberyards will be the most helpful source in matching the sizes of older panel products.

For those interested in the more technical aspects of plywood and other panel and engineered wood products, a visit to the APA Web site might be of interest. It's found at www.apawood.org. (