Signed in: dining as a guest in the clubs of St James's and Pall Mall

Between Piccadilly and The Mall lies what Londoners like to call the densest cluster of private members' clubs anywhere. St James's Street holds the oldest. White's, which began as a chocolate house in the 1690s, stands near the top of the street, with Boodle's, Brooks's and the Carlton a few doors below. Pall Mall is lined with the great clubhouses of the nineteenth century: the Travellers, the Reform, the Athenaeum on the corner of Waterloo Place, and others behind equally unmarked doors.

Most are closed to the public in every sense. You enter as a member's guest or not at all. For a visitor to London, an invitation to dine in one is a small honour and a slightly nerve-racking one, because the rules are real and nobody hands you a copy. Whether you are the guest of a member, or a member bringing a guest of your own, the same codes apply.

Every club has a hall porter, and every evening begins with one. Give your host's name rather than your own, and say that you are dining. You will be asked to wait in the hall or a side room until your host comes to collect you; do not wander further in to look for them. Coats and umbrellas are taken at the door, and anything larger than an evening bag is better left at your hotel.

Arrive on time, or a few minutes early. A late guest leaves the member waiting in his or her own club, which clubs dislike more than almost any other lapse. Many members prefer to meet a guest on the steps and walk in together, which spares everyone the hall altogether.

Every club differs in the detail, but across St James's the broad code is remarkably consistent.

Club dining rooms are among the loveliest rooms in London and among the least seen. Many are candlelit at dinner, with long windows, portraits of former members and silver that has been polished every evening for generations. The cooking leans classic: potted shrimps, a roast, a savoury, and claret laid down in the cellar decades ago and poured without ceremony.

Conversation is the point of the room. Voices stay low, tables are generously spaced, and nobody glances over. It is an exceptionally good setting for a dinner with someone new, because the room quietly asks both of you to be your most civil selves.

For a member, a club dinner with a companion is simply a dinner with a guest, and the club treats it as one. She is signed in like anyone else. What matters is that she knows the codes as well as you do, and at this level she will. She arrives at the time agreed, meets you on the steps or waits in the hall to be collected, and dresses for a candlelit room rather than a cocktail bar.

Tell Octavia which club when you write, and anything you know about its rules for lady guests. The companion she proposes will be one who enjoys evenings of this kind: someone who can talk to the member at the next table about architecture or opera and never let slip a word worth repeating. For formal dinners and occasions of every sort, the black tie plus one arrangement is written with exactly this in mind.

Some clubs will not suit. A few remain men only, some limit lady guests to particular rooms, and on some evenings a member would simply rather not introduce a new face to old friends. That is no reason to abandon the plan. The small hotels and old dining rooms of St James's sit a few doors from the clubs, and a dinner at Wiltons or The Ritz gives you the clubland evening without the clubland rules.

The reverse also works. Dine at your club alone or with colleagues, then meet your companion afterwards in a hotel bar nearby, never at the club door. Dover Street, where several newer clubs keep their rooms, is five minutes north across Piccadilly, with good hotel bars at either end.

Karolina, for whom discretion is second nature, is the first name Octavia thinks of for a club dining room; she will discuss cornices with the member at the next table and have forgotten his name by the coffee. Margo, one of our thirties companions, spends viewing weeks at Christie's on King Street, a street away from Pall Mall, and knows the tone of the quarter precisely. Sonya is calm enough to make a crowded club feel like a drawing room.

A club dinner sits comfortably inside the four hour dinner at £1,800. Other lengths are listed with our rates, and Octavia proposes a companion through booking as soon as she knows the club and the hour.