What is a "dead pin"??

It is a non-moveable pin in a tubular lock. Yes it functions essentially as a ward. It works because the front pins in a tubular lock rotate with the key.

How does the key rotate if the dead pin doesn't? How is the key cut to allow the rotation?

The dead pins does rotate along with the other pins.

The dead pin is a fixed part of the tubular lock and does not assist in locking the plug in place at all. If the key is cut to a depth in that position that is the height of the dead pin or deeper, the key can be seated fully to engage the other pins in the lock correctly. If the key isn't cut deeply enough in that position, full insertion of the key is impossible. If the plug is rotated, the dead pin rotates with it. BBE.

What about "dead latch" - or does that refer to the entire unit?

He's talking about in the context of a tubular lock.

Thanks, Put. Billy gave a good explantion of how it functions as a ward.

Yes

Yes it is, but it doesn't matter since the pin is dead and won't move in any direction. There is also a driver under it when it is in the locked position of the plug, that way when the plug is rotated more than 1/8th of a turn the live pins can still be active. BBE.

Just to add some texture to this old item, the "old" style Bell System pay phones (the ones with the nickle, dime, quarter slots across the top), frequently had the uppper portion of the phone secured with both the standard Western Electric manufactured lever tumbler lock, and with a Chicago Ace tubular lock that had a "female" threaded sleeve that screwed down on a "male" threaded rod that was affixed through the back of the phone. When the Ace key was rotated, the threaded sleeve advanced on the threaded rod and, in effect, screwed down the top housing, pulling the cover on tightly against the phone base.

That Ace key had four key-pull positions (12, 3, 6, and 9 o'clock), allowing fine adjustment. If I'm remembering correctly, the Ace cylinder contained a dead pin that was positioned between to of the regular Ace pin tumblers, thus preventing any Ace key that did not contain the warding cut from being inserted into the keyway, while maintaining seven pin security. A secondary benefit was the added leverage that the deadpin bearing on the key gave to the torquing action of tightening or loosening the bolt, relieving some of the torque on the spline (inside the Ace key) that usually provides the turning leverage on the plug.

R> The only definition I could find was "a pin which serves as a ward for a