How does a tongue-and-groove joint work on rubber tiles?

Tongue and groove is the joint used on the older Type NF rubber tiles, which WARCO no longer manufactures. The edges of these tiles are stepped: along two of them the lower half projects, with the groove in its top face, while along the other two the upper half projects, with the tongue on its underside. As they are laid, the stepped edges of adjacent tiles overlap and the tongue drops into the groove.

The joint ties the tiles to one another across the surface and keeps them from separating. Vertically, though, nothing holds the tongue down in the groove, so it can work its way out if conditions are unfavourable and the tiles shrink. You can lock the joint vertically as well, either by screwing through the stepped edge with stainless-steel wood screws or by bonding it with PU adhesive.