In England and in some parts of this country, if the younger hand
doubles it is a signal for his partner to lead the highest card of
his weakest suit; trusting that this weak suit and the doubler’s long
one will prove identical.
As this convention justifies a double with any established suit, it
multiplies the opportunities to double; but, on the other hand, since
the leader may have to choose between two weak suits, there is no
certainty that he will make the desired lead; he may instead strike
the dealer’s long suit with a disastrous result.