|Spoiler Inside: Solution to Puzzle||SelectShow>|
The rule has to do with points that end the letter. (The letter W has two points on top; the other three angles are not points that end the letter.) If the letter has one or more points that point down, the letter is in set 1. If it has one or more points but is not in set 1, it is in set 3. If it has no points, it is in set 2.
I is often printed with serifs even by people who do not otherwise put serifs on their printing – Hi, pleased to meet you – in which case it is in set 3 as given. If not, then it is in set 1.