Subdivision and armor coverage separate battlecruisers from battleships

Interwar capital-ship classification depended on more than speed or maximum belt thickness. Internal subdivision and the proportion of the hull protected by armor also help distinguish a dreadnought battleship, fast battleship, or battlecruiser. A battlecruiser might carry thicker armor over its vital area than a battleship while protecting less of the hull and providing less subdivision behind it. Fast battleships and battlecruisers also needed sufficient speed to maintain contact with the cruiser line. Comparing belt thickness alone therefore misses differences in hull construction and intended fighting role.