For leveling and plumbing, the best tool for most construction projects is a 4-foot carpenter's level, though a 2-foot version will cover most household projects like hanging shelves. Bear in mind that errors mushroom when you use a short level on a long board. On a 16-foot deck joist, a small, one-eighth-inch error over a 2-foot level will be magnified eight times into a glaring 1-inch mistake.
Laser levels that project a beam of light are the glitziest option, and a reasonable investment (about $30 for a basic model) if you do a lot of projects around the house. Of course, you have to level the level (often with some very small, built-in bubble vials) before it can shoot laser lines you can rely on.
In small spaces, a carpenter's square will show you true 90-degree corners. In large spaces, use geometry–the perfection of a 3-4-5 triangle. If you slept through geometry, here's how it works. Measure from a corner post out 3 feet along one wall, 4 feet along the other, and the hypotenuse (a line connecting the two measuring points) will equal 5 feet if the corner is square.
Levels and even plumb bobs are good for checking specific locations. But to survey a large area, say, the side of a house prior to re-siding, blocks and strings work better.
To find spots where a wall bellies in or out, cut three wood blocks the same size, tack one at each end of the wall and stretch a string tightly between them. Then use the third block to check the margin between the string and the wall, sliding it along to find the high and low spots.
On repetitive operations such as nailing clapboards or deck boards, it's not necessary to measure from scratch every time on each board. Instead, use a story pole—a long length of lumber on which equal spacing between boards is carefully marked. Still, it pays to measure back to your original marks, say, the edge of the deck, every few rows just to double-check your work.
Shims are sections of tapered wooden shingles, used opposed to each other (one sliding over another) to take up small discrepancies and make minor adjustments. They're commonly installed around windows and doors—increasing or decreasing the overlap to adjust the units into plumb and level position. Shims can't solve major framing blunders, but can fill in when an oddball timber is out of kilter.
Framing is forgiving, up to a point. Miss the mark by an inch or so and you have to pull some nails and try again. But when a stud is just a bit out of plumb, move it over with one or two toenails. That's the term for nailing on the side of a timber, near the end, at about a 45-degree angle.
The sideways force—plus a few extra whacks sometimes—can shift a stud just enough.
Once you maneuver key framing into the right position, lock it there with braces.