There are 28 pieces in a standard set of dominoes
There are 28 pieces in a standard set of dominoes.   Each domino has two numbers on its face.
The two numbers on 21 of the tiles differ.  These are referred to as singles or singletons.
The other 7 tiles repeat the same number twice on their faces.  These are the doubles or doublets.  In the figure below the 7 doublets comprise the right-most column.
The piece in the set with the most total dots is the 6-6 doublet, so sometimes a standard set of dominoes is called a 6-6 set.
The standard 6-6 set of dominoes . . .