I'm a fangirl obsessed with the 91-line. I mostly consider SHINee's Key, 2AM's Jinwoon, Beast's Dongwoon and MBLAQ's Mir the core members of the 91-line group mostly because the 4 of them are the ones often seen together. Usually, when I refer to the '91-line' it would just be composed of these 4 boys.
There are instances however, that these 4 boys are seen interacting with other members of the 91-line--such as Nicole of Kara, Jaejin of FTIsland, Min of miss A, U-Kiss' Eli was even seen holding hands with Key during Idol Sports Athletics; so there's a fair number of Idols born in the year 1991.
But, since it's usually these 4 boys who never fail to show their friendship and expose to the world how close they are, this site is dedicated to them. :)