Above is a ruler I bought at a college bookstore for a dollar or two and it has become one of my favorite tools. Since you can see through it, it is easy to draw a line and then draw another parallel line using one the lines on the ruler as a guide. To the left is part of the cover for Wild Honey from the Moon. I used the ruler to create all the boxes and corridors on the cover. It was one of my favorite parts of the book to illustrate. I later learned that rulers are made for just this kind of thing (see below).