I was just looking at the Sharingan in what I'm going to call your signature. (Whatever it's called, I'm new at this) I've always thought Madara's brother's eyes were really out of place. All other Mangekyo designs (except for Madara's eternal Mangekyo for obvious reasons) don't feature straight lines. The designs usually spiral outward in one way or another. I just thought it was an odd design choice on Kishimoto's part.
Having said that Shisui and Sasuke's eyes don't follow the three-pronged spiral trend. It would've made more sense if they did.
:hey: Luke, Welcome to NB! I hope you enjoy your stay here. also, don't forget to read the rules (this will contain almost all the basic details you need to know when you join and it is very helpful) and Good Luck!