  I discuss recent supergravity constructions of type IIB holographic
interfaces in four-dimensional ${\cal N}=1$ and ${\cal N}=4$ field theories. I
explain how each holographic interface can be compactified on a circle with an
SL$(2,{\bf Z})$ monodromy leading to a novel AdS$_4$ supergravity solution.
These AdS$_4$ backgrounds are argued to be dual to so-called J-fold
superconformal field theories in three dimensions.
