Ah, so only the white trousers are issued, to be worn as 3Bs. That is to say: 3Bs in the summer, but when else would one be wearing shortsleeves anyway?
Just to clarify some terminology about the navy uniform: everyone seems to refer to the black (navy blue, whatever) double-breasted jacket as a tunic--comparable with the army and air force's tunics. However, this is not correct because a tunic is actually closer to being a shirt rather than a jacket. A tunic, as the army has worn it, is a piece of clothing worn over an undershirt and underneath armour or protective clothing (like a parka or coat).
In our modern world we have two prime examples of tunics: (1) the CADPAT "Combat Shirt" is an example of a 'practical' tunic worn underneath webbing, combat parkas, etc. (2) the scarlet tunic of the light infantry is an example of a 'dress' tunic worn for ceremonial occasions and by RMC students. These are both worn over t-shirts and are not tucked in like other shirts--they somewhat resemble the modern suit jacket.
Since military service dress has modernised, the army's tunic has taken on the form of a suit jacket and has been worn over a collared shirt with tie. However it is still called a tunic because we in the military have a little thing called tradition (and just flip the notched collar and lapels of a suit jacket up and see if it doesn't resemble the form of a a high-collared ceremonial tunic).
After the 3 services unified, they wore a green suit modeled after the army's service dress. The jacket was, of course, called a tunic because of aforementioned tradition. When the environments got DEUs in the '80s, the navy got back something like it's traditional jacket, but I think that the name tunic was pretty powerful because it is still refered to as a tunic.
The real name for the navy's black jacket (besides COAT, BLACK A/W) is a 'reefer' jacket. The name comes from sailing--sails are reefed in, lines are reefed upon, etc. And its form comes not from a tunic, but from the frock coat and tailcoat of the eighteenth and nineteenth centuries. These coats, unlike tunics, were worn over dress shirts. Thus, the navy's service dress is the most like a modern suit and most definitely not a tunic.
The navy's tunic is the white, high-collared contraption that I mentioned earlier as a summer service dress uniform. This is worn over an undershirt and is meant as a cool alternative to the heavyweight black suit of the winter uniform.
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