apex rule

9 LETTERS · STARTS WITH A

apex rule is a valid 9-letter word worth 17 points in Scrabble and 20 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS

A1P3E1X8R1U1L1E1

1 + 3 + 1 + 8 + 0 + 1 + 1 + 1 + 1 =17 PTS

DOUBLE WORD → 34TRIPLE WORD → 51

MEANING

What does apex rule mean?