Page "Dominoes" Paragraph 16

Every tile belongs to the two suits of its two values, e. g. 0 – 3 belongs both to the blank suit ( or 0 suit ) and to the 3 suit.

Naturally the doubles form an exception in that each double belongs to only one suit.

In 42, the doubles are treated as an additional suit of doubles, so that, e. g., the double-six 6 – 6 belongs both to the 6 suit and the suit of doubles.