Sometimes you just have to get things straight. From ancient times, carpenters and other craftsmen have solved the problem of making things vertically straight with a simple weighted string called a plumb bob. The word plumb, meaning absolutely, comes from this reliable measure. Make a plumb bob from commercial or household materials to get things plumb straight at your house.
Buy a commercial plumb bob weight or create your own. Traditionally, a bob weight has a point at the bottom, which is very useful if you are spacing nails or brackets both above and below to set a beam or stringer. One carpenter suggests that a child's wooden spinning top has the needed point. Oval fishing weights do as well. Locksmiths often gives away key blanks that have been struck incorrectly; ask for a handful and include one large key to serve as the point on your weight.
Choose cord for durability and visibility. Thin nylon cord or heavy fishing line is usually weight-rated; look for cord or line that can be trusted with at least 4.54kg. of weight. Your bob will weigh much less, but you do not want it to snap while you are in the middle of serious work. Consider cord in a bright colour; it makes it easier to find in your tool box and easier to see as a vertical rule.
Tie one end of the cord to the weight. Make the cord as long as you need for most jobs; ceiling to 1 inch above the floor is a common carpenter's length. The weight needs to hang freely to keep your string line straight.
Tie an S hook or loop of wire to the other end of the cord, to minimise wear on the cord as you move it from place to place. You can tap a nail or brad close to where you are working and put the loop over it. Since the bob usually swings back and forth before coming to rest, a metal or wire hook prevents friction on the cord.