What the two words actually mean
In Britain, lounge suit is simply the formal name for an ordinary suit: a jacket and trousers cut from the same cloth, worn with a shirt and, as a rule, a tie. The phrase dates from the nineteenth century, when the lounge jacket was the relaxed alternative to the frock coat, and it has kept its old name long after the thing itself became the most formal garment most men own.
On an invitation it works chiefly by telling you what the evening is not. It is not black tie, so no dinner jacket. It is not morning dress, so no tails. And it is not the easy end of things either, so no open collar and no jacket over chinos. Guests from abroad sometimes read it as a request for something grander than a suit, or as a hint that anything will do. It means neither. It means the suit you would wear to an important meeting, worn with a little more care. Older invitations sometimes say dark suit instead, which asks for the same thing and leans, if anything, a shade more formal.
Tie or no tie
For a weekday dinner in W1, wear one. A host who wanted the room without ties would have written no tie or business casual on the invitation, and in the absence of either the tie is assumed. It is also far easier to take a tie off in a cloakroom than to find one at nine in the evening, and a man who arrives with one and removes it looks relaxed, whereas a man who arrives without one simply looks as though he misread the invitation.
Choose something plain or quietly patterned in silk, darker than the shirt, and knotted neatly without fuss. Leave novelty at home. If the host appears without a tie, you may follow his lead once dinner has begun, but not before; the reception is where first impressions are formed, and it is better to be the best dressed man in a room than the only one who guessed wrong.
Which colours, and which to leave in the wardrobe
Navy and charcoal are the two safe choices, and between them they cover almost every evening in London. Navy is a little warmer and a little younger; charcoal a little more formal and very forgiving under the light of a private dining room. Mid grey is excellent at lunch and reads slightly light after dark. Black is best avoided, since it tends to look either funereal or like the uniform of the staff, and it belongs, in any case, to the other dress code. Brown and tweed are for the country and the weekend, as the old rule about no brown in town still suggests.
A fine stripe is perfectly acceptable at a company dinner, though plain cloth usually looks better by candlelight. Wear a white or pale blue shirt, with double cuffs if you like them, dark socks long enough to stay out of sight when you sit, and black oxford shoes, properly polished. A plain white linen square, folded flat, is optional and never wrong. If you are having a suit made for the occasion, the letter on commissioning one explains how the fittings can be arranged around a working visit.
What the woman beside you will be wearing
For a woman, lounge suit on an invitation is usually taken to mean a cocktail dress or a well cut dress at the knee or a little below, or a tailored trouser suit worn with some polish. It does not mean a floor length gown, which belongs to black tie, and it does not mean daywear. A companion from this house dresses a shade under the room rather than a shade over it, which is the correct instinct at a company dinner and not one everybody has.
What matters most is that the two of you read as one party. That is a matter of register rather than of matching: a navy suit beside a dark, plain dress looks like two people who were invited to the same evening, whereas a navy suit beside something bright and long looks like two people who were sent to different evenings. Your companion will ask what you are wearing, the colour of the suit and of the tie, so that nothing clashes, and she will choose shoes she can stand in for three quarters of an hour at a reception without thinking about them.
Weekday dinners around Berkeley Square
Lounge suit is the code of the working dinner, and a great many of those in London are held within a few streets of Berkeley Square: in the panelled private rooms of Mayfair, in the dining rooms above the square, and in the smaller rooms off Dover Street and Albemarle Street, where a table of twelve counts as large. The bigger receptions go to the ballrooms of Park Lane, where black tie is more common and the invitation will say so.
An evening like this belongs to receptions and company dinners, and the dress code is one line in the short briefing that page describes. Most run to three or four hours, and four hours at £900 covers a reception, a dinner and the speeches with room to spare. Every length is set out on the tariff. The fee is given to her in person when she arrives, and it is never any part of the evening that your colleagues see.
Who to ask for
A room of colleagues is best met by somebody who has seen a great many of them, which is why I often suggest one of the circle's women in their thirties for these evenings. Nancy lives a short cab ride from the square, speaks slowly and deliberately, and is genuinely curious about whoever is seated beside her. Margo is direct, gives complete attention and forgets nothing she is told, which is a considerable asset at a table of names.
Sonya is the calmest company in the house, with a slow and authoritative English that makes a table of strangers settle around her. Any of the three will read the invitation once, dress to it exactly, and leave with nobody but you. The whole circle is in the Salon if you would rather choose for yourself.
One line of the presentations carries most of the names above, and that line is thirties and beyond.
Alongside it
Tonight
Put it to use
What this letter describes takes one written line to set in motion, and Booking shows how little there is to it.