Titanic's Olympic-class interiors had 16 primary watertight compartments. Fifteen transverse bulkheads divided the lower hull into these sections, allowing flooding to be isolated instead of spreading immediately through the ship.
The compartments formed part of the Olympic-class safety design shared by Titanic and Olympic. Watertight doors could be closed from the bridge, and the arrangement was intended to keep the ship afloat with up to four compartments flooded.
The system had an important limitation: the bulkheads did not rise high enough to prevent water from spilling over their tops as Titanic sank. Six forward compartments were damaged in the iceberg collision, exceeding the design assumption. Once the bow settled deeper into the sea, water cascaded from one compartment into the next.
A common mix-up is confusing the 16 compartments with the 15 bulkheads that created them. Olympic later received an additional subdivision during her post-disaster refit, bringing her total to 17.