There are 2 types of guitars which use two different types of strings

Steel stringed guitars have Slimmer necks and Nylon have wider necks

Which one do you use?

steel string

For Neck Widths Of 41mm to 46mm

& Some Smaller Nylon String Guitars

nylon string

For Neck Widths Of 47mm to 52mm

Unsure? Measure Neck Width At Nut