Surly Stainless Steel Chainring
These chainrings are more durable than the aluminium chainrings commonly found on bikes. They're perfect for the middle ring on a triple crankset, which often gets worn more quickly than the other rings (especially if the bike is mainly used in the city).
The first measurement in the size is the Bolt Circle Diameter (BCD) of your crank arm. The second measurement is the number of teeth.