How are "A Certain Magical Index" and "A Certain Scientific Railgun" connected?


According to MAL, the
[former](http://myanimelist.net/anime/4654/Toaru_Majutsu_no_Index) is the
parent story of the
[latter](http://myanimelist.net/anime/6213/Toaru_Kagaku_no_Railgun), that
appears described as a side story. However, I find no reference to these
questions:

**Do both stories take place in the same universe?  
Are they somewhat dependent on each other?  
If so, in which order should the series be watched?**




Chronologically, _Railgun_ comes first. Then near the middle of _Railgun_ (the
Level Upper Arc), the events and timeline start to converge with _Index_.

For an example, in episode 17 of the _Railgun_ anime, Komoe-sensei mentions
how she likes all her students, even the ones that blow up her roof, which is
probably a reference to when Index used Dragon's Breath (episode 6 of _Index_
).

The manga gives a better frame of reference, since it gives a exact date (July
16th). So the events in _Railgun_ begin 4 days before the Index novel (July
20th), where Touma first meets Index. Touma [meets](http://www.baka-
tsuki.org/project/index.php?title=Toaru_Majutsu_no_Index%3aVolume1_Prologue)
Mikoto on the eve of July 19th.

According to the [TV
Tropes](http://tvtropes.org/pmwiki/pmwiki.php/Manga/ACertainScientificRailgun?from=Manga.ToAruKagakuNoRailgun):

> The anime, however, violates this by adapting events from Volume 8 of the
> novels into the anime's first episode. As it stands, Chapters 18 to 39 occur
> around the time of Volume 3 in the novels, Chapter 40 is around volume 5,
> Chapters 41-42 and around volume 8 and Chapters 43-current take place around
> volume 9.




Chronologically, _Railgun_ comes first. Then near the middle of _Railgun_ (the
Level Upper Arc), the events and timeline start to converge with _Index_.

For an example, in episode 17 of the _Railgun_ anime, Komoe-sensei mentions
how she likes all her students, even the ones that blow up her roof, which is
probably a reference to when Index used Dragon's Breath (episode 6 of _Index_
).

The manga gives a better frame of reference, since it gives a exact date (July
16th). So the events in _Railgun_ begin 4 days before the Index novel (July
20th), where Touma first meets Index. Touma [meets](http://www.baka-
tsuki.org/project/index.php?title=Toaru_Majutsu_no_Index%3aVolume1_Prologue)
Mikoto on the eve of July 19th.

According to the [TV
Tropes](http://tvtropes.org/pmwiki/pmwiki.php/Manga/ACertainScientificRailgun?from=Manga.ToAruKagakuNoRailgun):

> The anime, however, violates this by adapting events from Volume 8 of the
> novels into the anime's first episode. As it stands, Chapters 18 to 39 occur
> around the time of Volume 3 in the novels, Chapter 40 is around volume 5,
> Chapters 41-42 and around volume 8 and Chapters 43-current take place around
> volume 9.

