be vertical

The technical term might be “vertical integration.”

The rear wall needs to be vertical rising up from the floor, but then at a certain height, it needs to start to gently slope toward the front of the fireplace.

Walls should be vertical, rooflines perfectly horizontal.

Two photons can be entangled so that each is in an uncertain both-ways state, but their polarizations are correlated so that if one is horizontal the other must be vertical and vice versa.

It can be vertical or horizontal and is meant to be a hub for your family life.