A mathematical equation used to find the youngest age a person can date without being considered a disgusting, cradle robbing freak. (A/2)-7=Y, where A is your age and Y is the youngest you should date.
Conversely the rule can be inverted to find the oldest age a person should date. 2(A-7)=O, where A is your age and O is the oldest you should date.
John is 26 and dating a 20 year old named Kate, (26/2)+7=20. Their relationship satisfies the half your age plus 7 rule.
Kate is 20 and dating a 26 year old named John. 2(20-7)=26. This relationships satisfies the inverse of the half your age plus seven rule.
The Abernathy Rule states that no matter what public event you attend (concert, movie, sporting event, etc.) there will be at least one idiot sitting around you who will ruin it by making noise, getting drunk, and/or just being an annoying jerk.
That drunk behind me didn't shut up the wholemovie, the Abernathy Rule was in full effect.