In the flesh wars that dictate the way we dress come summer, it's increasingly hard to know which bit of a body is seemly and stylish to put on parade. Too much chest - particularly if your boobs are suspiciously buoyant and conversational - and people will inevitably assume they're fake; even if your chest is all your own work, the rules on cleavage display as one ages are complex.
It's all too easy to cross the line into crepery. Or tartiness. Or, worse case, both.
Hmm, you think. How about upper arms? Well, too much arm and you enter the Madonna arena, where you are obliged to lift the perfect size of weight the perfect number of times in order to achieve faultlessly toned biceps and not a hulking great upper body like a barn door. So what else? You do, after all, need to show a morsel of flesh - it is summer we're talking about. But which morsel?
This year, heaven defend us, there is even talk of a return of the bare midriff. Plenty of catwalks, from YSL to Prada (a pairing that could hardly be more potent), showed the look, and you know it's only a matter of time before Cheryl Cole allows an enviable slice of honey-coloured tummy to see the light of day. Not on your nelly, I hear you cry, ready to give up and crawl back under the duvet of despond. But stop. There is a place I know, a place you could go. And I'll bet yours can pass muster with the best of them.
I'm talking about the Lower Thigh Zone, or LTZ for short. This is the sassy piece of leg which ranges from just above the knee and upwards for three or four inches, not much more. This zone has, of course, long been a centre of attention and affection; there is something wickedly flirtatious about a hand placed lightly on the knee or just beyond, the suggestion of what lies ahead, the hidden unknown (it's one reason why men so love the idea of stockings and suspenders, which make a big play of the thigh, enhancing it with a now-you-see-it-now-you-don't appeal).
These days, showing an extra bit of leg may seem a trifle vanilla, but it's worth remembering that until the 1990s it was rare to see a woman over 40 flashing this still-intimate portion of flesh. Now, though, few don't. Wags, royals, academics, popstrels - they're all at it, for work, for weekends, forever, sometimes bare-to-there, sometimes housed in opaque tights, but always one naughty notch higher than a decade or so ago. A hem that halts above the knee has somehow emerged as a uniform for us all, transcending fashion trends to become the bottom line in almost every woman's wardrobe. It's the hem that's hijacked Britain! You see the LTZ everywhere - getting out of taxis, settling on to bar stools, cruising around the shops on a Saturday afternoon, picking up the kids from school.
We've become so used to seeing that handspan of flesh on parade that a lowered hem, even to the knee, now looks blue-stocking and uptight.
As you will already have noticed, spring fashion takes the LTZ very seriously indeed. The catwalks were overrun with endless, endless legs: hugely high heels, wild hosiery, pencil skirts that stop an inch or three above the knee to send your legs out into the waiting world simmering with hotty promise. It's a slap in the face for hemline theory, which suggests that we recessionistas ought to be schlepping about in maxi skirts and bemoaning the state of our pension pots. But as if to buck all that bull, the spring catwalks showed a lot of leg. That special slice of thigh showed up at Balmain, McQueen and Chanel. There were sexy Parisienne princesses at Louis Vuitton showing off that same gorgeous space. Everywhere else - from Roland Mouret to Stella McCartney, from Christopher Kane to Giambattista Vialli - the message was that, this spring, you really need a skirt. Quite a short skirt. A Mad Men skirt. A ladylike skirt, hovering above the knee and making a play of your LTZ.
Today's obligatory red carpet ensemble is, of course, shorter still - a shrinky mini and killer heels (as seen on Gwyneth Paltrow, who launched her "comeback body" a few months back, using those mile-long legs as twin power pistons). Really, everyone's at it. If you're 18 and gorgeous, you'll probably be pulling the Pixie-Agyness-Daisy-Lowe leg look, all thigh top and high-tops, all opaque tights, bovver boots and a T-shirt for a dress. And if it's not a screamingly high-line skirt, the same job is done by skinny-fit leggings - those shiny ones that cling to a leg as if they're dipped in treacle, the too-tight sweaters of the day. Amy Molyneaux, fashion designer for PPQ, says with a shrug: "I always get my legs out. And all my friends get theirs out too, even if they don't think they've got conventionally perfect legs - they wear tiny skirts and leggings or coloured tights in mod style."
The glory of all this, as anyone who has embarked on a project of wearing fashionably shorter skirts will know, is that raising your hem by even a marginal amount will make your legs look longer. If your hem hits precisely the right point, your legs will look thinner too. Longer. Thinner. Such a triumph of slim with very little effort on your part! Better still, as Molyneaux says, legs needn't be pin-thin to look cracking. Like a face, a good leg requires something in the way of padding. "Take comfort in the knowledge that what makes a good pair of legs is their shape, not how skinny they are," says personal trainer Matt Roberts. "Great legs have curves in perfect measure: not too much muscle on the thigh (too athletic), and no excess fat around the knee or ankle (too tree-trunk). Good legs have definition."
What's more, and here's the joyous heart of the matter, you can access all this at pretty much any age. Though some old-timers might argue that skirt length should extend with age, what's happening in fashion right now flips all that. Over the past few years, hems have crept ever higher, whatever the vintage of the legs beneath. As anyone past 40 will know, this very particular piece of thigh can still walk the walk as the years tick by. In a recent poll of "best celebrity legs", Elle Macpherson, aged 45, came first. Kylie Minogue (40), Cindy Crawford (43) and Elizabeth Hurley (43) took second, third and fourth places... You only have to look at Tina Turner, Bette Midler, Whoopi Goldberg, Michelle Obama, Sophia Loren, even Cilla Black, to know that great legs are great legs for decades; while everywhere else requires hoiking and lifting and pushing up that hill, the legs dance on. Yes, you may gaze upon your body, checking off the tyrannies of time... past your mother's hands and your crumpled cleavage... past the softened belly and the slump rump... but then you arrive at your legs. Blimey. Still in good nick, you think. Better get 'em out.
"The legs are the last to go," agrees Emma Soames, editor-at-large of Saga magazine and in her late 50s. "The last time I wore a skirt below the knee was Christmas night 1999. Now a skirt that length seems so ageing. I don't even own a skirt that reaches below the knee. I would rather be boiled in oil. You do have to get the right length for your age though. My rule is if people are shocked that I look much younger from behind than from the front, then the skirt may be a smidge too short."
At 76 Joan Bakewell wouldn't give a long skirt house room, far less actually wear one. "I don't want to wear sensible skirts," she says. "I don't want to look like an old frump.
I mix with active working people, so I don't want to look like I have come from a pensioners' meeting. My life is somewhere else. My skirts and dresses hover around the knee."
As Soames and Bakewell will know, however, the LTZ is a subtle spot. You can't just crash about and hope to get it right, particularly as the years roll by. Personally, as I arrived at 40, I found that it is the nuances of style that matter. God really is in the details. And the minutiae of the hemline debate suddenly became live in a way that wouldn't even have registered on my radar a decade ago. Back then, my fashion statements were general and wantonly extreme: too-tight tops, perilous heels, shrunken jackets, outsize shirts - anything that broadly captured the mood of the moment. Now, though, everything hinges on the particular. And the precise spot where a skirt ends, how much leg you reveal, becomes critical. It all boils down to proportion. Get it right, and it's flesh without flash. It's exposure with added class. The quest, then, is to find your sweet spot, the place where a hem loves your legs the best. This will depend largely on your very own legs, but there is a Golden Rule for this, a mandate so useful that it may well change your very fashion future, so listen up: Never Cut a Hem Through the Fattest Part of Your Leg.
Not rocket science, I know, but it's surprising how many of us get it wrong, thus screwing an outfit before it has seen the light of day. So mid-thigh, mid-knee and mid-calf are all wildly wrong. To find your own hemline hotspot you could experiment a bit. Grab a pillowcase and hold it in front of you, starting with it draped to the floor. Watch your mirror carefully as you gradually raise your pillowcase. Up, up and - wahey - there will be a point when your legs look slimmest and angels will sing. But don't, whatever you do, stop two inches south of the knee...
"There is that Tory-wife, just-below-the-knee length that is very ageing and unflattering," says Sarah Sands, a newspaper executive in her late 40s. "I call it the Norma Major length. I would never wear that length. I have lived in shortish skirts all my working life - in the 80s, in my first job, I was power dressing in snappy little skirt suits. Now I am wearing a similar look but with slightly lower heels - whisper it - for comfort." (Interestingly, you may well see Tory wife Samantha Cameron pulling the same stunt.) Vogue writer Christa D'Souza, also in her late 40s, knows exactly how to work the look: "My uniform is a denim mini and Converse or Birkenstocks," she says. "Always flats. I think heels and minis are too much once you are over 30. If I do wear heels with a mini, I won't wear make-up or brush my hair."
Hosiery is, of course, another neat way to skirt the age issue. You'll have noticed the coloured opaques doing the rounds, the It Tights in the shops, the crazy patterned hose clambering up a leg like ivy up a telegraph pole. Chanel is doing fine business in its £100 tights, despite the recession taking a bite out of most budgets. Opaques are selling like mad because they make a display of LTZ effortless and dependable, a canny compromise in the perennial balancing act between siren and slut. You still get the seductive curve of a calf, the suggestive expanse of leg heading north.
Says Mary Portas: "I stopped going bare-legged and started wearing short skirts and dresses with tights when I turned 40. I'm obsessed by getting hosiery right. Hosiery is so important; people underestimate that. Getting the right colour, the right sheen, the right denier... But I love the short skirt, tights and ankle-boot thing. It really works for me. I'll be persisting with that one for a while." At 43 Sadie Frost has also taken to tights: "Until about three years ago I didn't mind it if I inadvertently showed a bit of knicker, but I do now. I am happy with my body but also aware that I'm getting older. My children are older. My sons get stressed out by it. So these days I don't wear bare legs that much; I wear tights or leggings." PPQ's Amy Molyneaux, by the way, contends that women ought to "up the denier of the tights with age", which seems a good rule of thumb (though when you hit support stockings, you may want to give your LTZ a rest).
Finally, do bear in mind that the more leg you show, the less chest you need to advertise. Masses of breast served up with your LTZ will turn you from a subtle, shape-shifting, age-defying demi-goddess into a trollop. On this we are all agreed.
When to call a halt to your skirt...
For early 20s
Use the "does your cooch touch the seat" rule. Similarly, if your skivvies show when you walk upstairs, you've gone too short. Your dad will already have told you this.
For under 30s
Stand up straight, arms at your sides. Your hem shouldn't stop higher than your fingertips.
For over 40s
Try the bend-over rule. Pick up something from the floor. Anything. A raisin will do. If your skirt reveals your cellulite, ditch it.
For prudes
Kneel. If your skirt touches the floor, you're fine, really.
For true believers
According to Pure Fashion, an American faith-based programme that encourages teen girls to live, act and dress in accordance with their dignity as children of God, skirts "should not be shorter than four fingers above the top of the kneecap". See? Even God loves the LTZ.