Rules are sometimes not easy to understand and few people can remember all of them, but if you decide to race, you should understand Rule 18 which is one of the most common rule you will be faced with in any race, so enjoy.
ROUNDING AND PASSING MARKS AND OBSTRUCTIONS From the International Sailing Federation Racing Rules of Sailing 2001-2004
In rule 18, room is room for an inside boat to round or pass between an outside boat and a mark or obstruction, including room to tack or gybe when either is a normal part of the maneuver.
18.1 When This Rule Applies Rule 18 applies when boats are about to round or pass a mark they are required to leave on the same side, or an obstruction on the same side, until they have passed it. However, it does not apply (a) at a starting mark surrounded by navigable water or at its anchor line from the time the boats are approaching them to start until they have passed them, or (b) between boats on opposite tacks, either on a beat to windward or when the proper course for one or both of them to round or pass the mark or obstruction is to tack.
18.2 Giving Room; Keeping Clear
(a) OVERLAPPED – BASIC RULE When boats are overlapped the outside boat shall give the inside boat room to round or pass the mark or obstruction, and if the inside boat has right of way the outside boat shall also keep clear. Other parts of rule 18 contain exceptions to this rule.
(b) OVERLAPPED AT THE ZONE If boats were overlapped before either of them reached the two-length zone and the overlap is broken after one of them has reached it, the boat that was on the outside shall continue to give the other boat room. If the outside boat becomes clear astern or overlapped inside the other boat, she is not entitled to room and shall keep clear.
(c) NOT OVERLAPPED AT THE ZONE If a boat is clear ahead at the time she reaches the two-length zone, the boat clear astern shall thereafter keep clear. If the boat clear astern becomes overlapped outside the other boat she shall also give the inside boat room. If the boat clear astern becomes overlapped inside the other boat she is not entitled to room. If the boat that was clear ahead passes head to wind, rule 18.2(c) no longer applies.
(d) CHANGING COURSE TO ROUND OR PASS When rule 18 applies between two boats and the right-of-way boat is changing course to round or pass a mark, rule 16 does not apply between her and the other boat.
(e) OVERLAP RIGHTS If there is reasonable doubt that a boat obtained or broke an overlap in time, it shall be presumed that she did not. If the outside boat is unable to give room when an overlap begins, rules 18.2(a) and 18.2(b) do not apply.
18.3 Tacking at a Mark If two boats were approaching a mark on opposite tacks and one of them completes a tack in the two-length zone when the other is fetching the mark, rule 18.2 does not apply. The boat that tacked (a) shall not cause the other boat to sail above close-hauled to avoid her or prevent the other boat from passing the mark, and (b) shall give room if the other boat becomes overlapped inside her, in which case rule 15 does not apply.
18.4 Gybing When an inside overlapped right-of-way boat must gybe at a mark or obstruction to sail her proper course, until she gybes she shall sail no farther from the mark or obstruction than needed to sail that course.
18.5 Passing a Continuing Obstruction While boats are passing a continuing obstruction, rules 18.2(b) and 18.2(c) do not apply. A boat clear astern that obtains an inside overlap is entitled to room to pass between the other boat and the obstruction only if at the moment the overlap begins there is room to do so. If there is not, she is not entitled to room and shall keep clear.
Rules are sometimes not easy to understand and few people can remember all of them, but if you decide to race, you should understand Rule 18 which is one of the most common rule you will be faced with in any race, so enjoy.
When boats are overlapped the outside boat shall give the inside boat room to round or pass the mark or obstruction, and if the inside boat has right of way the outside boat shall also keep clear. Other parts of rule 18 contain exceptions to this rule. 
