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The grandfather rule, in sports which usually only permit participants to play for the team of their native country, is an exception which gives participants the option to play for the country of any of their ancestors up to the grandparents. Despite common name for the rule, grandparents of either sex can be invoked equally and it is sometimes referred to as the grandparent rule or the granny rule.
a Player may only play for the [Team] of the country in which: (a) he was born; or (b) one parent or grandparent was born; or (...)
The term "grandparent" is given a narrow interpretation:
the relevant grandparents for the purposes of establishing a Player’s eligibility pursuant to Regulation 8.1(b) will be the Player’s blood grandparents. It is not possible under Regulation 8.1(b) to assume eligibility via non-blood grandparents even if a Player has been formally and legally adopted. For the avoidance of any doubt, stepparents and fostering parents will not be considered to constitute a Player’s parent for the purposes of Regulation 8.1(b)
This exception is also incorporated directly into certain national regulations which govern the club-level teams from that country, as shown by regulation 22.214.171.124 of the Irish Rugby Football Union:
The following are the registered players who are eligible to play in the All Ireland League and Cup: (i) A Player who qualifies to play for Ireland under I.R.B. Regulations. (...)
if, in addition to having the relevant nationality, he fulfils at least one of the following conditions: a) He was born on the territory of the relevant Association; b) His biological mother or biological father was born on the territory of the relevant Association; c) His grandmother or grandfather was born on the territory of the relevant Association; (...)
Effects on teams
A controversial rule, sports fans often debate whether this rule weakens smaller countries by creating the temptation for the best players to abandon their native country and declare elsewhere in the hopes of greater glory, or if on the contrary it helps smaller countries by giving them access to a broader range of players who wouldn't typically be considered to be picked for their country of birth.