Martin MA130 Folk Silk & Steel Guitar Strings can be used on either Classical or Acoustic guitar. They have a high tension compared to a nylon string and a low tension when compared to a steel string. A very versatile string, they have the drive of a steel string yet can also have the soft tonal properties associated with classical strings. Great on acoustic guitars for people that have soft fingertips or for wanting a more mellow sound. Great on ACOUSTIC guitars when you want the sound of classical strings, but want the drive and power of steel. The string thicknesses are 11 1/2, 14, 23, 28, 38, 47.