40 Singles - The singles rating refers to the weight of the thread and the tightness of the weave that is used in the garment. Most traditional t-shirts generally have a 20 singles rating.
40 singles is a superior quality thread weight that features a tighter weave than both 20 and 30 singles and will feel much softer and finer to the touch. The bottom line: 40 singles fabric is more comfortable to wear.