Board Thread:Theories and Speculation/@comment-25645405-20150717154745/@comment-5407586-20150718072024

Choku Tomoe in the end were mentioned only by Madara, and in one case he stated that he and Sasuke were the only ones to gain the Straight Tomoe. That makes me think it's something related to being Indra's transmigrant (though Indra's Mangekyō looks nothing like a Straight Swirl) and/or to the Eternal Mangekyō Sharingan. Maybe to both, seeing how only Indra's transmigrant gained the eternal light, to our knowledge.