Submarine context;
Conning towers were originally small vertical cylinders with windows. The idea
was that the conning tower could be poked above the surface to allow the submarine
to see where it was going, and to locate targets. Because the conning tower was
much smaller than the main hull of the submarine, it was harder to see.
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By WWII periscopes had been developed, and the conning tower had become the place
where one looked out of the periscope for navigation or attack purposes. The conning
tower still served the purpose of keeping the main hull deeper in the water, but
by now this was done mainly to avoid being seen from the air. A submarine in this
position is known as being at periscope depth.
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Now that sonar is the primary attack sensor, conning towers are no longer designed
into new submarines.
The terms conning tower, fairwater, and sail are often (incorrectly) used
interchangeably. | 
