Page "Mass of Paul VI" Paragraph 52

From the middle of the seventeenth century, almost all new Latin-rite altars were built against a wall or backed by a reredos, with a tabernacle placed on the altar or inserted into the reredos.

This meant that the priest turned to the people, putting his back to the altar, only for a few short moments at Mass.

However, the Tridentine Missal itself speaks of celebrating versus populum, and gives corresponding instructions for the priest when performing actions that in the other orientation involved turning around in order to face the people.