Dbl: Undefined onesuiter. Shows some strength since partner can pass. 
2C : Clubs + another suit. 
2D : Diamonds + a majorsuit. 
2H : Hearts + Spades. 
2S : Spadesuit. Has not enough strength to double. 

Woodgroves Commentaries. I don't like this system, but if my partner sticks a gun to my head, I play it with this changes: 
2C : Clubs + a majorsuit (4 card suit). 
2D : Diamonds + majorfarge (4 card suit). 
2NT: Both minor. 

Since it often is better to play 5-2 in a major suit instead of 4-3 with a minor suit, should the minor suit be at least 5 card. With a good 5 card major suit, you should double if you have some strength, even if you have a 4/5 card minorsuit.