Part of an axoneme consisting in a doublet microtubule. Nine of these outer doublets form the 9+0 axoneme, while the 9+2 axoneme also contains a central pair. Dynein arms attached to the doublets provide the mechanism of movement of the cilium. [ http://www.ncbi.nlm.nih.gov/pubmed/5044758 http://www.ncbi.nlm.nih.gov/pubmed/5664206 http://en.wikipedia.org/wiki/Axoneme ]