Drawing a LineDrawing a straight line on a computer is one of the easiest things you can do in art software. Most programs have a tool for drawing a line, and many of those have an additional feature that allows you to make the line perfectly horizontal, vertical or diagonal with ease. Even if there's not a tool specifically for making lines, there's usually a way to make a perfect line without any fuss.
Unfortunately, drawing in real life is not quite so easy. The human hand just isn't designed for drawing straight lines. If you stop to think about it, because of the way we hold writing implements like pencils, we make curved lines instead of straight ones. The problem here is that we're used to writing things by moving only the wrist. What we're actually doing is drawing small curves by pivoting around a fixed point.
In the above animation, the hand draws a line by only moving at the wrist. While the red line next to it is perfectly straight, the new line is very curved.
There are three solutions to this problem. Use whichever technique works best for you.
The first and easiest is just to make your line out of many small lines. Make one small line, then move your hand aside. Begin the next line at the end of the first line, and continue to move your hand as you draw until the line is drawn.
The second trick is to place dots along the path the line will take and then simply connect them. This works best for distances under a few inches, as you can better control where the line ends and begins. You can check your dots are in the right places by placing your pencil so that one end is at the start of the line and the other end is at the end of the line. If any dots are out of place, they won't be on the edge of your pencil.
The third and last trick is fairly obvious, though I've been hearing that some young artists don't realize this is an option. You can use the side of a book, ruler, or other straight edged object to draw a straight line. Position one end of the flat side at where you want your line to start, and the other end of the object where you want the line to end. Then trace the edge of the object to draw a straight line. Rulers and the flat side of a protractor are the best choices for this, but other things work in a pinch. Just be sure that the side of the object is flat on the page and doesn't have a lip. You don't want to draw under the object; you want to draw against the object.