Refer to figure.
A runway designation marking is mandatory at the threshold of paved runways and advisable at the threshold of unpaved runways, such as grass runways. Typically, it consists of a two-digit number, but if parallel runways exist, it may be augmented with a letter. On a single runway, dual parallel runways and triple parallel runways the two-digit number shall be the whole number nearest the one-tenth of the magnetic North when viewed from the direction of approach.

The authorized runway designators are depicted in the accompanying figure. (Source: ICAO Annex 14)

A: Standard pattern
B: Parallel pattern
C: Optional pattern
D: Not approved