Nylgut copper wound (D)
To keep the calculation simple and allow the use of any available string calculator, strings are indicated by the letter D followed by a number giving the corresponding equivalent diameter of a plain gut string.
For example, "D140" for a string means that it corresponds to a gut string with a diameter of 1.40 mm. The actual diameter of the string is of course smaller, but under playing conditions it has the same working tension as a gut string with a diameter of 1.40 mm.
