How the granny vest became sexy — and why you should leave the boxy, boyfriend styles to your daughter

Cardigans have an image problem. Play word association and you get grandma, prim, two, school uniform. No one ever automatically thinks: Cool! Glamorous! Or even classic.

It comes down to this: people think cardigans aren't sexy. The young think (or they did) that they are as attractive as a jersey wrap dress; and women like us think they will make us look like our mothers in their twinset days, or just homely, cozy, scruffy. Anyway, fashion toast.

Well, that's all about to change. In fact, it has already changed. If you've been looking closely, you'll have noticed that Brad Pitt was recently spotted in Beverly Hills with his girlfriend Ines de Ramon – and what did she wear on their date night?

Yes, a cardigan! A khaki cardigan, mid-length, luxuriously soft (probably mohair and cashmere), bell sleeves, edge to edge, over a white fitted top, leather pants and boots. There is a lot of exposed collarbone visible.

We can look back on this as the moment when the long cardigan – worn open over something slimmer and revealing a bit of skin – became ideal for date-night wear. Sexy but soft and relaxed, and surprisingly elegant. Although De Ramon is thirty and doesn't need to cover her butt, that could be a bonus for the rest of us.

Star power: Jennifer Garner wears a cropped navy blue and white cardigan with jeans and ballet flats

Mango does a longer cardigan in cream (£29.99, shop.mango.com) and Marks & Spencer's even longer, almost midaxi, cappuccino cardigan (£129, marksandspencer.com) could do a similar job – worn over a tonal turtleneck, with pants, it has casual dinner, if not date night, written all over it.

The point is that from now on cardigans are meant to be worn in an elegant and/or sexy way in the evening, or to add a touch of chic in the office.

So how do you ensure you stay on track and don't end up in schlump territory? Simple. Think of three types of vests and follow these rules:

Date night cardigan

Apart from the buttonless long cardigan (above), the perfect evening cardigan is prim and sexy. It is a round neck, falls on the hips, soft (not cotton, that looks too thin) and fitted but not skinny.

For a cashmere classic, try M&S (£89). For a plain, boxy, woolen style, there's & Other Stories (£65, stories.com) or Arket (£67, arket.com).

You wear your prim and sexy unbuttoned to almost cleavage, and with the sleeves turned up. (The big secret to making the cardigan look fresh and modern is to wear it on bare skin, and not over a thick shirt). This year the cut-off long one-shoulder sweater (remember the '80s?) is back and this is the grown-up, less flashy alternative. A cardigan with an off-shoulder look, that is what we strive for.

Do not: go for a V-neck. They are much harder to wear because you have to consider what goes underneath. Or a wide boyfriend style. Those oversized varsity style cardigans are very fashionable, but leave them to your daughters.

Taylor Swift wears a black oversized cardigan with brown knee-high boots

Alternative jacket

This one has pockets and gold-plated or jeweled buttons, like a Chanel knitted jacket. Or it's simple yet structured and boxy. Mango has a good style with black and white stripes with bling-like buttons (£49.99, shop.mango.com) or one in a black spotted tweedy knit (£35.99).

There are other structured cardigans that can replace blazers: Zara has a plain, light gray crew-neck knitted cardigan, fastened with a single silver button at the neck (was £35.99, now £12.99, zara.com) that a sleek matching sweater looks stylish.

At the other end of the scale is Zara's boxy faux fur cardigan with floral buttons (£45.99), worn with a black straight skirt, belt from office to evening.

Do not: go for too many decorations. The virtue of a cardigan jacket is its simplicity – and you can always spice it up with jewelry, a bag chain or a scarf.

The smart vest

You can get around the 'no V-neck' rule if it's an extra high V-neck and a smart shape, like Toast's wool 'neat cardigan' (£145, toa.st). If you're looking for something to wear with a skirt, this one hits just at the hip. Thanks to its neat shape and wide ribbed hem, it is perfect to wear over a slightly flared model. Add a knotted silk scarf and you have a whole new look.

Do not: Wear shirt collars underneath. A crew neck or hidden layer top is best.