In earlier times, composers/arrangers had to write down music notation on a piece of paper with a quill pen (which was very messy because ink would spill and smudge all over the paper). Mistakes often had to be scratched out. The only way to make a copy of the music was to write it out by hand. Sometimes the copyist was not very accurate, and there are examples of the same piece of music copied out with different harmonies or notes in a melody. Now composers/arrangers can use music notation software to write a piece of music. They can enter in the notes one by one, or connect their midi keyboard to their computer and enter in the notes by playing them. Once complete, the music can be saved as a pdf file and published for sale online.