The Victoria & Albert Museum – London’s best if you are interested in style and decoration – is running an exhibition on the Golden Age of Couture, 1947-1957. The exhibits of vintage Dior, Balenciaga and Balmain gowns are fascinating: the relentless tightening of waists, the details of how dresses were cut and the style culture of Paris.

But the discovery of the exhibition for me was in the gift shop: a new print of Hardy Amies’s ABC of Men’s Fashion. First published in 1964, it contains a staggering number of sayings about men’s fashion that have remained in circulation. The most famous of course is: “A man should look as if he had bought his clothes with intelligence, put them on with care and then forgotten all about them.”

There are also little gems such as “Cummerbund. ‘Anglo-Saxon 1616. A sash or girdle worn around the waist’. Need I say more?” And “Fancy waistcoat. Fancy is a suitably unpleasant word for a rather nasty article.” Although Amies does go on to say “I should exclude from this fancy waistcoats worn with a morning coat. Here a touch of dandyism seems to be desirable.”

For the purposes of Permanent Style, however, the most interesting aspect of Amies’s book is his declarations about the fashion of the sixties, and his confidence in its practicality and longevity.

The sixties ideal is long and slim, with very narrow trousers, a four-buttoned suit and ankle boots. His tie is thin and he wears a trilby pushed forward on the head. Everything is about length and height, all buttoned-up and tight. His guidelines are:

Trousers: “Trousers follow the natural contours of the leg as closely as comfort and the fall of the cloth permit.”

Jacket: “The line is accentuated by the use of three buttons as fastening rather than two or one. Four are better still.”

Revers: “These higher fastenings automatically shorten the length of the revers and it is natural that they should also become narrow.”

Sleeves: “Sleeves should be as narrow as comfort permits, and at the wrist should just encompass the cuff of the shirt.”

Waist: “A four-button fastening looks well with a jacket cut rather straight at the sides. This looser, straighter jacket has for some time been liked by the young. It looks casual. The older find it comfortable.”

Headwear: “Given the slightest chance, young people rush to put on a hat. They are designed to be worn well forward on the head, thus enhancing the forward lines of the rest of the costume.”

Footwear: “Elastic-sided boots are more comfortable to wear, easier to put on, nicer to look at, and better integrated with the rest of one’s clothes than the lace-up kind. They seem to have just about everything in their favour.”

I find it fascinating that someone so revered now and then had such confidence that style was changing. It is an illustrative lesson in assumptions about so-called classic style or indeed permanent style.