The balls used in international competition are close to 40mm in diameter and made of either celluloid or a non-celluloid plastic. Beginning this year (2014) all ITTF pro tour events will use the non-celluloid plastic ball. Balls are graded in "stars" -- with the best quality balls (in terms of sphericity and uniformity) given a grade of "* * *" or 3-stars, 3-star balls are used in tournaments and in league matches, while 2 and 1 star balls are often used for training. 3-star balls are more expensive than the lesser quality balls.