Four front row players would entitle them to have six reserves and five (or more) front row players means a team can have seven reserves.
At any time in a match, a team may object to the referee about the number of opponents they are playing.
If it is more than fifteen, the captain of that team has to reduce the number appropriately.
Play would restart with a penalty at the place where play stopped but the score at that time would remain unaltered.
In looking at player movements on and off the field, it is important to distinguish between "temporary" and "permanent" replacements.
Temporary replacements are just that - temporary - there to cover a player who has left the field to have a Blood Injury attended to, or a Head Injury Assessment.
They can also be on the field to provide cover for a front row player who has been temporarily suspended for whatever reason (foul play and repeated infringements usually) and leave the field when that player comes back on the field.
A player may be a temporary replacement for any of these three situations as many times as necessary during a game.
Permanent replacement though is when a player comes onto the field because of injury to a team mate.
The injured player cannot return to play under any circumstances.
However, if a player has been replaced tactically by another player, this is considered to be a "substitution".
Substituted players are also considered as permanent replacements but they can return to the field under one of the three specific situations listed above, but only if they didn't go off the field because of injury.
For a Blood Injury, players can be temporarily replaced if they suffer a wound where the bleeding is "active and uncontrolled".
They must leave the field to have the bleeding controlled or covered but this must take no more than fifteen minutes actual time, otherwise the replacement player becomes "permanent" and the temporarily replaced player cannot return to the field.
For first class matches such as Heartland, ITM Cup, Super Rugby and test matches, a player may also be temporarily replaced if he has to leave the field to have a Head Injury Assessment by a medically-trained person.
The WRFU appoints a local doctor to carry out such assessments for Heartland matches.
Such players have ten minutes actual time in which to be assessed, but if they cannot return to the field before time expires the "temporary" replacement becomes "permanent" and that player may not return to the field.
If the assessment starts within ten minutes of half time, the player being assessed must return to play at the start of the second half, otherwise he has been permanently replaced. The same applies to Blood Injuries.
A front row player who is replaced tactically can return for the same two reasons, but can also come on to replace a frontrower who has been temporarily suspended.
In this case, another player must leave the field also so that the team still only has 14, so those front row players get more privileges than others in the team!
New Zealand rugby lost one of its great servants with the death of Sir John Graham this week.
Besides being an All Black and later administrator, he was a teacher at several schools and headmaster of Auckland Grammer for a number of years.
Perhaps his most famous quote was that education was more important than sport.
While I was attending Christchurch Boys' High School, Graham was a history teacher and at his playing peak.
The school benefited from this as all All Black teams of the era came to the school to train, so we got to see the likes of Graham, Colin 'Pinetree' Meads and Don Clarke up close and personal.
I only attended one class with Graham as I was more into geography than history.
He had just come back from the very successful 1963-64 tour of Great Britain and France and spoke to the boys who were lucky enough to be accredited University Entrance (remember that?) while the others sat their UE exams.
The rugby purists among us hung on his every word as he told us about rugby and touring in foreign countries but, oddly enough, like most young adolescents the only bits I can recall now related to his impressions of prostitutes in France.
The words temporary, permanent and substitution never applied in those days.