Crash
Over opponents 1 NT opening:
Double = 2 suits of the same color (reds or blacks)
2C = 2 suits of the same rank (majors or minors)
2D = 2 suits of the same shape (D and S or C and H)
2M or 3m = natural
Different variations are possible.
Note that this method is now illegal under the revised ACBL general convention chart.