Watertight markings are placed on doors and hatches, to indicate to Jack and Jenny weather the door/hatch can be left open, or even if they can go through it. All this depends on the watertight condition of the ship, as set by the command.
There are three types of markings, in the form of letters and these are X, Y, and Z, always pronounced phoneticaly, X-Ray, Yankee and Zulu. What follows is a general guide only, and will vary from ship type to ship type.
X-ray doors/hatches are found in the lower part of the ship, generaly along and below the waterline, rising above the waterline at the stern and bow of the ship. Identified with a big black 'X' on the door/hatch.
You can go through them, as long as they are shut and fully clipped afterwards. To be left open for any reason, a special tally is required from HQ1/SCC. These tallies are strictly controlled.
KEPT CLOSED: At all times, no matter the watertight condition.
Yankee hatches/doors are usually a deck or two higher above X-Ray hatches. Usualy kept closed for convienience, some will be left open if they lead down to a mess or other commonly used area. Identified with a big black 'Y' on them. You can go through them at will.
When the ship is in condition yankee, all X-Ray and Yankee hatches/doors to be shut and fully clipped.
Some yankee hatches will have a smaller 'hatch within a hatch' called a kidney hatch. This kidney hatch may have a higher watertight marking than the actual hatch it's in.
KEPT CLOSED: When the ship is in condition yankee or zulu.
Zulu doors and hatches are the remainder of the watertight doors throughout the ship. Identified by a big black 'Z', these doors will be on main thoroughfares and busy areas in the higher parts of the ship. Condition Zulu is only usually set when at war at action stations. As a lot of these doors see a lot of traffic, some in the busier areas, will have a marking // opposite two of the clips. When the two clip rule is in force, then only these two clips may be used to keep the door shut to ease passage. They can be a pain in the arrse when in AWD.
These doors are usually open at all times.
KEPT CLOSED: When the ship is in condition zulu.