Made of Solid Sterling Silver. Innovative oval design holds your thumb well without pinching the sides like traditional thimbles.  To size your thumb: Wrap a piece of dental floss around the widest part of the knuckle of your thumb. Make a mark on the floss where two strands of floss pass or meet each other to get the circumference of your thumb . Measure the distance between the two marks. 5.2cm = T20; 5.6cm = T21; 6cm = T22; 6.5cm = T23; 7cm = T24; 7.5cm=T25. The thimble fitting is very forgiving - everyone can use two different sizes. When your measurement falls between two sizes, pick the larger one.