A guitar pick is a plectrum used for guitars. Picks are generally made of one uniform material, such as some kind of plastic, rubber, felt, tortoiseshell, wood, metal, glass, tagua, thermosetting plastic or stone. They are often shaped in an acute isosceles triangle with the two equal corners rounded and the third corner less rounded. They are used to strum chords or to sound individual notes on a guitar.

A Reuleaux triangle is a curved triangle with constant width, the simplest and best known curve of constant width other than the circle. It is formed from the intersection of three circular disks, each having its center on the boundary of the other two. Constant width means that the separation of every two parallel supporting lines is the same, independent of their orientation. Because its width is constant, the Reuleaux triangle is one answer to the question "Other than a circle, what shape can a manhole cover be made so that it cannot fall down through the hole?"