Sandpaper letters are a Montessori material used to teach children how to form and recognize letters. They are made of thin sheets of sandpaper glued to cardboard, with the letters of the alphabet raised on the sandpaper surface. Children can trace the letters with their fingers, which provides a tactile experience that helps them remember the shape and formation of each letter. Sandpaper letters are an important tool in helping children develop their handwriting skills and an understanding of the alphabet.