Meaning of nine plus two array

nine' plus" two' array"

Pronunciation: [key]
— Cell Biol. Cell Biol.
  1. the arrangement of microtubules in a flagellum or cilium, consisting of a ring of nine evenly spaced couplets surrounding two central singlets. Symbol: 9 + 2.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.