Bertie went to barbecues? 
I’ve understood it to mean the full top hat,white tie and tails, as for a formal dinner. Black tie was still relatively informal, surely?
(Compare and contrast Fred Astaire, and the Dowager Lady Grantham ticking off someone wearing black tie - “Why are you dressed as a waiter?”)
Black tie is always “relatively informal” compared to white tie.
But black tie certainly existed in the time in which the Wooster novels are set, and indeed before. Therefore, there were functions to which black tie was worn. And the fact that the short coat often worn with black tie is called a “dinner jacket” gives us a clue as to the kind of functions involved.
White tie was worn only to the grandest and most formal functions; black tie to evening functions a degree or two below that. A ball or a dinner dance with a large number of guests, or given by a very high-ranking member of society, would be a white tie function; white tie was also worn to the theatre or the opera. But a dinner with no dancing, especially if it was a smaller function, or associated with a club or society, or a dinner in your own home or the home of a friend, would more likely be black tie.
See the picture at this link to Harcourts, a robe-maker in Canada. The tabs/bands are the white strips in place of a tie, falling over the top of the waistcoat.
The Court of Appeal of Maryland still wears the tabs/bands, with red robes.
Since we seem to have branched out into the historical vagaries of gentlemens’ dress, I thought it worth a mention that Field Marshal Wellesley, 1st Duke of Wellington, a leader of fashion in his time, was refused entry to Almack’s, the most exclusive private social and dancing club in London, when he arrived in trousers (a daytime form of dress) rather than the regulation pantaloons (de rigueur for evening).
Known in ecclesiastical circles as “Geneva bands”, indicating a minister/priest is of a Protestant rather than Catholic persuasion.
What’s the difference? I always assumed they were synonyms.
Trousers go to the ankle. Pantaloons end just below the knee, with the gentleman wearing skin-tight stockings to display his calves.
Most of these gents are wearing pantaloons.
That’s a bit … bright!
Back at home, our last remnants of old school court dressup is that judges wear Spanish-style robes: black open-collared with satin facings, and white lace on the cuffs (but no badges or collars as in Spain, and no distinctions of “rank”: fancier lace or just means you had more money to spend); lawyers and prosecutors had to be robed until around the 1970s or 80s.
I am suitably impressed that a world-spanning empire was assembed by people who minded stuff like this.
Black tie is indeed formal nowadays, they would note 'white tie" instead of ‘formal wear’ if that is what they wanted.
Essentially, the American Constitution was drafted by men in shorts 
Ah, thanks! Never knew that is what those were called.
In some circles, those might be called “knickerbockers”, or “knickers”, but British vernacular has sufficiently co-opted at least the latter to mean panties that that usage is rare.
Which might explain why it feels rather like the US is on the High Way to Hell.
Note as well that pantaloons were more upper class than breaches or knickerbockers. Pantaloons would typically be made of a good quality “superfine” cloth, and could be brightly coloured, like the blue-grey that Ben Franklin is wearing in the picture.
Knee breaches could be the workaday wear of labourers and middle class folk, but more durable, made with wool (like tweed).
Most Australian jurisdictions still wear wigs and gowns at trial level.
In Courts of Appeal and the High Court, judges wear robes but no wigs; counsel still wig up, unless counsel come from a state like Victoria which has fully de-wigged.
“Everyday” wigs for barristers and judges are short and not uncomfortable (assuming air-conditioning, which has sometimes been problematic to fit efficiently into old historic courthouses).
They feel like a well-fitting hat more than anything else, but they do wear with time and perspiration.
That patina of age is part of signalling experience to more junior barristers.
Full-bottomed wigs are more of a pain. They drape over the shoulders and the front of the chest, and wearing them is a ponderous experience. You can’t casually turn your head to speak to someone because the long wig does not want to turn with you, because of the draping thing I mentioned. You also can’t hear properly because they cover your ears and create a sort of echoing effect that is hard to describe, which is why they are only used on formal occasions.
Wigs are not powdered, by the way.
When wigs were in general use, they typically needed to be sent out to be re-curled regularly, by hot irons. A firm called Ede and Ravenscroft in London developed a technique of manufacture that made recurling unnecessary, and now they are still the best formal (as opposed to theatrical) wig makers in existence.
You send your measurements taken across different parts of your head to them (or visit them in London) and they send you a wig with an impressive hand painted tin with your name on it.
When I got my short wig (over 30 years ago now) it cost about $1500.
When I got my full bottomed wig (about a dozen years ago) it was nearly $10,000. My late father paid for it as a present; no way could I then afford it otherwise without drastic steps like selling the kids for medical experiments.
Thanks to you’all.
Ta-dah: Wiki “bands (neckwear.)”
Which tells ma aka “preaching bands” and “barrister bands.”
An attention to detail will get you far in this world. 