Some people when putting on a new set of guitar strings or bass guitar strings grab the string in the middle and stretch it out. This is a very bad idea. Guitar strings are made out of high carbon steel or nickel, these metals are not meant to be stretched out. When you pull on the guitar string, you make the string thinner in the middle and fatter on the ends. Stretching the guitar string this way will cause your guitar to not play evenly in tune on all your frets.