Page "Pai gow" Paragraph 9

So for instance, a 1-3 tile ( a tile with one pip on one end and three pips on the other, for a total of four pips ) used with a 2-3 tile ( with five total pips ) will score nine, since four plus five is nine.

A 2-3 tile with a 5-6 tile will score six, and not sixteen, because you drop the 1.

And a 5-5 tile with a 4-6 tile will score zero, since ten plus ten is twenty, and twenty reduces to zero when the tens place is dropped.