The term on-center often abbreviated "OC" or "o.c.," is commonly used in construction drawings, architectural plans, and woodworking designs. It indicates that the given dimension applies to the distance between the center of one member to the center of the next member.
For example, in framing a residential stud wall, a building plan may call for the wall studs to be placed every 16 inches OC against the floor plate and top plate. This means that the center of each stud is placed at intervals of 16 inches. OC measurements are important, as building codes may have particular stipulations for the intervals of framing members, such as studs, rafters, and floor joists.
Why Use On-Center Spacing?
The usefulness of on-center spacing is best demonstrated by the common practice of stud framing, or "stick framing." A framed wall is usually covered by a sheet good, like plywood or drywall. Sheet goods most commonly come in 4 by 8-foot sheets, meaning they are 48 inches wide. If you frame a stud wall at 16 inches OC or even 24 inches OC, the edge of a vertical sheet will fall over the center of a stud (48 is divisible by 16 and 24). This ensures that the edge of the sheet is fully supported and is backed by about 3/4 inch of wood for nailing or screwing into. The next sheet to be installed gets the same amount of support and backing.
Another reason to use on-center spacing is that it ensures accurate layouts with variable thicknesses of material. Most milled lumber does not come in precise dimensions. A 2x4 should be 1 1/2 inch thick, but often they are 1 3/8 inch or 1 9/16. Using on-center spacing, it doesn't matter if the material thickness varies a bit, because your layout is based on the center of the material, so the difference is split equally to both sides.
That said, framers usually lay out stud walls so the edges of studs (not the centers) align with the layout marks (because the material is standard enough for this application). But if you need to hit the center of a member regardless of its thickness, on-center spacing does the job.
Tip for On-Center Layouts
There's an easy way to ensure on-center spacing using the special markings on a standard (non-metric) tape measure. Most tape measures use red type, an arrow, or other special markings for intervals of 16 inches. This is because house walls and floors are typically framed with 16-inch OC spacing.
For a basic 16-inch OC layout on a stud wall, you can simply make a mark every time you see a red number: 16, 32, 48, 64, 80, 96, etc. Then, you place the studs on the same side of each mark; if you measured from left to right, the studs would go on the right side of each mark.
However, if you're starting the layout at a corner or end of a wall and you want the first stud to be completely covered with a sheet good (let's say drywall), it works a little differently. The drywall sheet is 48 inches wide, but because the first edge of the sheet is covering the first stud, rather than falling on the center of it, the span from the outside edge (not the center) of the first stud to the center of the third stud must equal 48 inches. Therefore, you must take out half the thickness (3/4 inch) of the first stud from the beginning of the layout. An easy way to do this is to mark the second stud at 15 1/4 inches, then move the end of the tape measure to that mark. Pulling from that mark, you can mark the rest of the studs at every 16 inches.
If this sounds confusing, grab a tape measure and try it. It's easy to get once you've done it for real.