The rule of 20 is a guideline of opening a suit at the one level. If the number of HCPs plus the total length of the longest two suits is at least 20, open it, otherwise, don't open.

For example, a 4-3-3-3 shape needs 13 HCPs to open, while a 6-5-1-1 shape only needs 9 HCPs to open.

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