Legal pads are among the most famous types of notepad. They got their name because they have been used by lawyers for decades, although many other consumers use them as well. The reason for the yellow color of classic pads is actually rather mysterious, although there are several competing theories. One suggestion is that the yellow color is more "stimulating" and makes dark ink written on it easier to read. Another theory is that the color was chosen to hide the age of the paper, or the fact that it was made of recycled materials.
Technically, the only requirement for a true "legal pad" is that it must have margins which are 1.25 inches (3.17 centimeters) from the left hand side. This margin leaves room to make notes or comments. These pads can come in all sizes and colors, but most people think of yellow pads with letter-sized tear off sheets, attached at the top with gum rather than stitches or spirals.
According to the American Pad and Paper Company, a man named Thomas Holley conceived of the idea of using sortings, the scraps of paper left after cutting it into sheets, to make small notepads. Previously, the sortings had gone to waste, and Holley's idea was well received. He started a company that began producing notepads in an assortment of sizes.
Lawyers adopted the pads because their profession requires a great deal of writing and note taking. With time, notepads with letter size paper came to be known as legal pads because the pads could be easily placed into client files. Oddly enough, legal sized pads are not as common as they once were, due to an effort to keep legal paperwork at a standardized size. The guiding lines help to keep writing even and straight and, apparently, the larger margin was added by special request from the legal community.
How the color yellow came to be associated with these pads is a matter for some debate. The American Pad and Paper Company says that yellow was chosen because it was more intellectually stimulating. Dark print on light backgrounds is certainly easier to read, and yellow paper is less likely to create painful glare than white paper. It is also how difficult to tell how old the paper is, since paper yellows with time. Alternately, the paper may have been uniformly dyed yellow to help conceal the fact that it was made from an assortment of recycled material.
Whatever the original reason, legal pads are commonly associated with the color yellow. Many legal firms have actually switched to white pads, however, because yellow paper cannot easily be recycled. Some people also prefer the brighter contrast of dark ink on white paper. It seems unlikely, however, that the yellow paper will ever vanish entirely.