Most programming languages allow you to add notes to your programs that are ignored by the computer. This lets you explain what you've done to someone else who might read your program later. In QBASIC we use the apostrophe (') to begin a comment. Here's an example:

 ' A program to draw boxes all over the screen
 ' This is a comment, QBASIC will ignore it
 SCREEN 12
 CLS
 ' Draw 50 boxes
 FOR I = 1 TO 50
 ' Pick the location of the box
 X1 = INT(RND * 640)
 Y1 = INT(RND * 480)
 X2 = INT(RND * 640)
 Y2 = INT(RND * 480)
 ' Pick the color for the box
 Color1 = INT(RND * 16)
 ' Draw the box
 LINE (X1, Y1) - (X2, Y2), Color1, BF
 NEXT I

The computer will ignore all those comment lines, but us humans can read them and remember how a program works. Good programmers use comments to help others understand what they have done. Comments can also help us remember what we did when we come back to a program after working on something else for a while.

Constants

Another way to make your programs easier to understand is to use constants. Constants look and act like variables, but they cannot be changed. Here's a useful program:

 CONST Pi = 3.141593
 INPUT "Enter the radius of a circle: ", Radius
 PRINT "The circumference is:"; 2 * Pi * Radius
 PRINT "The area is:"; Pi * Radius * Radius

If we didn't use the constant Pi, we would have to copy the number 3.141593 two places in the above program. Using a constant makes the program easier to read and understand. It also keeps us from making mistakes when copying.