The Martin MM13 Retro Monel acoustic guitar strings, in medium / light gauge 13-56, are made from a nickel-based alloy blend to create a quickly mellowing string for vintage tones. These strings will bring out unique woody tones from your acoustic that you've not heard before! If you like that old-school, vintage characteristic then these are the strings for you!ÿIn Bluegrass gauge 13-56 these string a powerful bass end with a loud tone and great resonance. Great sounding strings! The individual string thicknesses are: 13, 16, 26, 34, 44, 56."I never thought I'd see the day when I could put Martin Monel strings back on my D-28. Welcome back, old friend, I've missed you." - Tony Rice.