The phrase relaxes one half of the room
Black tie optional is a host’s way of saying that the evening is formal and that nobody will be turned away at the door for wearing a dark suit. Both answers are correct. A dinner suit with a black bow tie and a well-cut dark suit with a tie will sit at the same table without either man looking as though he misread the card.
What the phrase does not do is relax the women’s half of the room. It is not an invitation to jeans for anyone, and it does not mean a long gown is expected. A cocktail dress, or a long dress if the house prefers one, is the shape of the answer. Reading the phrase as permission to dress down entirely is the one mistake that shows.
If a man takes the suit
A dark suit in charcoal or navy, a shirt that is not the brightest white in the room if the suit is soft, and a tie in silk or wool rather than a novelty. Shoes polished, not new: a new pair announces itself. Cufflinks only where the shirt has the holes for them.
Where the evening runs late and the room is warm, the jacket stays on until the host removes his. A jacket removed first by a guest is a small thing that reads as a large one, because it tells the table that the guest has decided the evening is over before the host has.
If a man takes the dinner suit
Black dinner suit, black bow tie tied by hand, a white shirt with a turn-down or wing collar, and no waistcoat unless the invitation says so. Patent shoes are correct and plain black oxfords are equally correct. A long tie with a dinner suit is not black tie; it is evening dress worn halfway, and the halfway is visible.
The dinner suit is the safer choice where the invitation comes from a house that entertains formally and the phrase is used to spare guests the expense of hiring. Where the card is issued by a company, a club or a hotel, the suit is usually the intended answer.
The details guests ask about
Never a pocket square chosen to match a partner’s dress; white linen, folded plainly, or none. No visible watch at black tie if the evening is properly formal, and no jewellery that catches the light at table. Scent is worn for a distance of one hand, not a room.
A woman’s answer: a cocktail dress with a shoe she can stand in for four hours, one piece of jewellery rather than three, and a wrap or coat that can be left with staff without anxiety. If the invitation names a colour or a theme, the theme is a request, not decoration to be improved upon.
When the card is ambiguous
Cards are written by people, and some are written badly. Where the wording does not settle the question, the method is to ask the host’s secretary or the person who issued the invitation, once, briefly, and to accept the answer without negotiating. Asking once is correct. Asking twice is a way of telling a host that their evening is now a problem to be solved.
Where there is nobody to ask, dress to the higher of the two readings and arrive unbothered. A guest slightly too formal in a room that turned out to be relaxed is a small cost. A guest visibly underdressed in a formal room is a cost the host carries all evening.