User blog comment:EmirK/Who do you want for the main antagonist in Toaru Kagaku no Accelerator?/@comment-17714224-20131023235948/@comment-5230978-20131025110454

Yes, between vol 19 and 20 no arc could occur, but they could before vol 12. After all Accel was out during vol 8, Toaru Majutsu to Kagaku no Ensemble, the movie, Railgun chapter 43, the first day of Daihasei and in the first chapter of his manga. Furthermore, considering that an arc can occur even in a single day (most of the arcs where Accel is involved as protagonist don't last more than one day) there is enough room for 4-5 arcs, if not 6. The canonicity wouldn't be at risk, unless the author or JC Staff (with a possible anime adaptation) decide to mess with it.