Should the dealer turn the trump down, the eldest hand has the privilege of
making it what he pleases, and the rule to be generally followed is, if
possible, to make it next in suit, or the same colour as the trump turned.
The reason for this is evident. If the trump turned should be a diamond,
and the dealer refuse to take it up, it is a fair inference that neither of
the bowers is in the hands of your opponents; for if the dealer's partner
had held one of them, he would in all probability have assisted; and the
fact of the dealer turning down the trump also raises the presumption that
he had neither of them. Then, in the absence of either bower, an otherwise
weak hand could make the point in the same colour. For reverse reasons, the
partner of the dealer would "cross the suit," and make it clubs or spades;
for, his partner having evidenced weakness in the red suits, by turning a
red card down, it would be but fair to presume that his strength was in the
black suits.
Be careful how you make the trump when your adversaries have scored three
points, and, as a general rule, do not make or order up a trump unless you
are elder hand or the dealer's partner.