In part 1 the use of the double to penalise over-bidders was explained. However the most used purpose for doubles is the so called “take-out double”

A bid of a double does NOT raise the level of the auction

If opponents open in a SUIT at the 1 level, use the double to ask partner to bid their longest suit. The best hands are 4-4-4-1 hands with a singleton in opponent’s suit, but frequently you will have to double with 4-4-3-2 shapes.

Conditions for a double: an opening hand, SHORTAGE in opponents bid suit and a choice of suit.

Partner MUST respond to your double, unless there is an intervening bid. So just bid your best suit, with a jump if you hold more than 8 points

If the opponents have bid 2 suits then a ‘double’ implies that you hold the other two suits.