Board Thread:Questions and Answers/@comment-30241447-20190327004433/@comment-25364614-20190327135134

For the Index Light Novels, arcs aren't typically named. In the past, for novels whose arcs have been left unnamed, the wiki holds a poll to decide what fans think is the best name for the arc. Its worked well for the most part.

For Railgun and Accelerator manga: they have always been named eventually. The current Accelerator manga arc is the only arc that has yet to be named.