The six words that set a room
Six phrases do almost all the work. White tie means tails, a white waistcoat and a white bow tie, and it is the only code obeyed exactly. Black tie means a dinner suit, a black bow tie and a plain white shirt. Morning dress means a morning coat, striped trousers and a formal tie, worn before six in the evening. Lounge suit means the darkest ordinary suit a person owns, with a tie unless the card says otherwise. Smart casual means a jacket and trousers that belong together, and smart day dress is the same idea in a dress. Anything beyond these is a description rather than a code.
The words are not a ranking of guests. They exist so that forty people arriving separately look like one room, and a person who meets the words exactly has done everything the host asked. That is why a dinner suit worn to a lounge suit dinner reads as a small comment, while a plain suit at a black tie dinner reads as bad luck.
What optional permits
Black tie optional means two things are correct: a dinner suit, or a dark suit with a tie. A woman may wear a long dress or a short one, and neither corrects the other. What the word does not mean is optional attendance at a standard, and it does not invite a third option that is cheaper, brighter or more comfortable.
Where a guest can meet neither, the courtesy is to meet the spirit: dark, plain, covered and quietly made. Turning up deliberately underdressed to show independence asks the room to look, and being looked at is not the same as being at ease.
Reading the invitation for the rest of it
The words on the card are only half the instruction. The hour sets the register: anything before six in the evening is day dress, whatever the words say. The ground sets the shoe, because gravel, grass and a long walk from a car are their own dress code, and a garden in June will find out a thin heel.
The rest is in the line the host did not need to write: a wedding at noon under canvas, a dinner in a restaurant that states nothing, a christening at eleven in a cold church. Read hour, place and season together, and the sentence on the card becomes an instruction rather than a puzzle.
Where the same words mean different things
Practice varies by country, and the variation is real. In London black tie means a dinner suit; in parts of the United States it may mean the same thing worn with more latitude in the accessories. In Paris an invitation that states no code expects a dark suit, and a guest who telephones to ask will be answered kindly. In Dubai and Singapore the heat and the room adjust the cloth even where the words are unchanged.
In all four places one rule holds: a question asked early is a courtesy, and a question asked at the door is a problem. Ask the host, or the host’s secretary, once, and accept the answer without negotiating it.
When nothing you own meets the words
Say so to the host, once, plainly: you have no dinner suit and want to be sure you will not be out of place. Almost every host will answer that a dark suit is perfectly fine, and will be glad the question came early. What hosts mind is not the absence of the suit but the surprise of it at the door.
Then borrow, or have one thing made plainly and keep it for years. A house that holds one good version of each register is never in difficulty again: day, evening, travel, mourning, pressed and repaired and ready. That is a wardrobe rather than an expense, and it answers this question for a lifetime.