.041 

A folk guitar string purchased individually that draws .41 is a string with a gauge of .41 inches, or about 1.04 millimeters.

Folk guitar strings generally range in gauge from .010 to .013 inches (about .25 to .33 millimeters) for the treble strings (the thinner strings), and .050 to .070 inches (about 1.27 to 1.78 millimeters) for the bass strings (the thicker strings).

Thus, a folk guitar string with a gauge of 0.41 inches would be very thick and probably used for the lower strings of the guitar. It is important to note that the tension of the string can also affect the way it sounds and plays, so the choice of string also depends on the player's personal preferences and the style of music they play.