Well, a subterranean gallery was dug, and on both sides these compartments were hollowed out, one above the other.

The bodies of the dead were laid in them, for the most part simply wrapped in shrouds.

Then the aperture was closed with tiles or marble slabs, carefully cemented.

So, as you can see, everything explains itself.

If other families joined the first one, or the burial association became more numerous, fresh galleries were added to those already filled.