Dome on horns (see Wikipedia)

The squinches are four arches placed at an angle in the four corners of a square room or tower. These arches support four small slanted walls which transform the square into an octagon, on which an octagonal dome can therefore rest, or a small circular dome by the use of fittings or by the addition of small additional trumpets.

The technique of cupola on horns is one of the two main techniques (with the dome on pendants) which allows a dome with a round or octagonal base to be suspended above a square space which circumscribes it. The trumpet is the most primitive and simplest technique to achieve, but it only allows small domes to be suspended and requires good thickness of wall on the sides of the square (these walls can themselves rest on arches to distribute the weight on four pillars). The pendant, on the other hand, is more complex to make, allows the diameter of the dome to be considerably enlarged and does not need a wall on the sides of the square. (..)