There are three different thickness families of NACA Airfoils and the thickness functions are explained on the following pages:

NACA 4-digit airfoils use the 4-digit thickness distribution functions.

NACA 5-digit airfoils use the 4-digit thickness distribution functions.

NACA 16-series airfoils use the 4-digit-modified distribution functions.

NACA 6-series and 6A-series airfoils use the 6-series thickness distributions.