30/1 (or 30 Singles) refers to the yarn count. This is the unit of measurement that defines the fineness of the thread. The higher the number, the finer the thread and thus a finer, softer fabric is created. T-shirts made of 30/1 are much softer and have a better drape than shirts made of 20/1. The terms are used several ways, i.e. 30s, 30/1 and 30 Singles. They all mean the same.