We wear the trousers, we’ve burned the bras, the high heels are strictly optional and nobody under 50 has even heard of a Playtex 18-Hour girdle, so how is it that a century after female suffrage we still lack an essential element of post-patriarchal dress?

Ladies, there’s an elephant in the changing room: the inside pocket. This is the pocket of liberty, the pocket of financial independence, the pocket that no man on earth would ever be without in his jacket. It’s designed first and foremost for safe stowage of a wallet. Yet 99 per cent of jackets made for women do not have one. (And, designers, don’t complain that women have breasts. Men’s bodies are no more streamlined in their own way and it didn’t stop trousers being invented.)

For stowing serious stuff, women are expected to drag a bag around, something external to the body, prone to be left behind; something, to my mind, as hobbling as five-inch bootees. I have nothing against five-inch bootees, but for everyday, you want flats. And an inside pocket. But while sneakers are everywhere, an inside pocket is still a chimera in female fashion. Only one designer has ever understood this. Helmut Lang.

Susan Irvine

Understandably, though unbearably, Lang gave up fashion for art in 2005. I still have my Helmut Lang man-cut (and yet not) suit from the late 1990s, the jacket of which boasts not one but two inside pockets. This is like all your buses coming at once if you are into pockets. I can slip wallet, photo, smartphone, key, fountain pen, emergency chocolate and more into these pockets; I can saunter down the street hands-free as well as every other kind of free. That’s lib. And on PJs? At first glance, an inside pocket on your PJs is a bit of a lobster telephone. Why would you need an inside pocket on a garment you will be wearing not only indoors but in bed? But think again. What could be more intimate?

In this pocket, you will stow the one thing you must always have about your person. OK, condom if you must. But I feel it should be something closer to your heart. Especially if it goes in the inside pocket of the inside pocket — which, remarkably, the Gentlewoman/Tekla pyjama also has. This inner chamber could shelter your migraine pill, a lock of your little boy’s hair, Rilke, a scrap to write your dreams on.

Some years ago, my husband, who is Jewish, gave me a slender gold ingot in a tiny tartan envelope. Put that in your shoe, he told me, for when you have to run. I didn’t put it in my shoe; I keep it by my bed. But that’s not good enough. Because when the thought police, or whoever it’s going to be in the future, come knocking, you need to jump without thinking, out of the bedroom window and down the road, supplied with love, poetry and the ultimate cashable object all in one sliver stowed next to your heart. That’s what I’d keep in the inside pocket of the inside pocket of my PJs. Whatever you plan to keep in yours, make sure that in one way or another it’s worth its weight in gold.

Susan Irvine is the author of Muse and Corpus and Perfume. She is a visiting lecturer at the Royal College of Art in London. Extracted from Modern Manners: Instructions for living fabulously well by The Gentlewoman, Phaidon, £19.95; www.thegentlewoman.co.uk.