The effect of temperature changes on the pitch of a stretched string of constant length is presented for both the flexible and stiff string. It is shown that the main contribution is through a tension change, but temperature variations of Young’s modulus, coefficient of linear expansion, and radius also produce secondary effects. Numerical examples are presented for a steel wire in a piano. A relaxation of constant length is treated for the flexible string, and the relative importance of the temperature-dependent parameters is discussed, including an entropy analysis.