The rule of 15 is a guideline of opening 1♠ at the fourth seat. Add the number of HCPs and the number of ♠s, if it is at least 15, open 1♠. This rule comes from the fact that ♠ is the highest suit, thus opening it prevents the opponents coming in cheaply.
For example, the following hands qualify for a 1♠ opening at the fourth seat under this rule:
| ♠ | A732 |
|---|---|
| ♥ | KQ3 |
| ♦ | 952 |
| ♣ | Q76 |