What are the conditions for valid congregational prayer in a mosque, other buildings and open fields?

Summary

This guideline sets conditions for congregational prayer inside mosques with one or several floors, multiple buildings within a mosque, non-mosque buildings and open fields. The main condition is that followers know the imam's movements by sight, by seeing the rows, via a screen or by hearing a relayer of the takbir. Where followers are separated, the distance from the imam or other followers must not exceed 150 metres (300 cubits). Stairs between floors must be inside the building, and doors nailed shut before prayer invalidate it, while doors merely closed or locked do not.

Written in our own words from the original fatwa — check the source before acting on it.