Board Thread:Naruto Discussions/@comment-1872728-20160329033053/@comment-2005101-20160430090745

If it referred to the literal shape of the Mangekyō Sharingan's Tomoe Seal, then it would have been Izuna, rather than Madara, that was known to be among the only two Uchiha along with Sasuke to awaken it, as there was nothing "straight" about the Tomoe Seal of Madara's original Mangekyō Sharingan.

Keep in mind that "straight" can also be interpreted as something aligned, correct, or perfect. The Eien no Mangekyō Sharingan has also been called a "perfect" Mangekyō Sharingan.