The official guidance at the start of the royal tour of Canada was that Kate would be seen and not heard. I wonder if Prince William was advised to be heard and not seen, for he has been edited out of most of the newspaper photographs.

The tragedy for him and so many men is that, although they may rule the world, they are not that interesting to look at. The Duchess of Cambridge, on the other hand, is a creature of endless variety. At the moment she is particularly keen on wearing white – why not repeat a success? – but there will be different phases to come. The life of Diana, Princess of Wales, is poignantly told through her wardrobe. Naive and Sloaney, hit-and-miss husband-pleasing, sophisticated revenge dressing, global fashion elite.

The fountains and foundations that celebrated her legacy were all very well, but what reminded people of her most were her sons and her dresses. Last month, a Victor Edelstein dress that Diana wore to dance with John Travolta fetched £510,000 at an auction in Toronto.

So the most useful study aid for the Duchess's Canadian tour is probably WhatKateWore.com. Its predictions have been admirably thoughtful. For Kate's wreath-laying the betting was on "formal attire, expect a dress and possibly a hat". At the youth barbecue, on the other hand, "look for a more casual style". What do you reckon for the LA Bafta black-tie reception? Yes, you are getting the hang of it: Kate is going to put on an evening gown. Unless she loses her marbles and wears the "bright red J Brand 811 jeans" we know she has bought for this trip.

There are many constitutional quibbles about the role of the royal family. Primogeniture, for instance. But nobody doubts that it is the duty of women to parade in a variety of outfits that are noteworthy and chic, while also being frugal and respectable. What a tightrope! The Queen is more of a role model in this than Princess Diana, who was sending out too many mixed messages through her wardrobe. The Queen is not emitting a cry for help: she sets out only to be smart and visible.

I joined an intimate group of 8,000 people, at a Buckingham Palace garden party last week. Wonderful to see the grass and foliage. But would the sniper on the palace roof take aim at me if I hitched up my skirt and scampered up a tree to get a glimpse of Her Majesty? Miraculously, I found a spot behind a wheelchair and got a clear view of a tiny figure in beige and lilac. As WhatElizabethWore .Com might say, "formal attire, expect a dress and certainly a hat".

Our head of state rides a merry-go-round of hats, gloves and jewels. It is a duty she shares with all public wives. Men make speeches; women wear dresses. Even if women make speeches, they are judged by their dresses. Just read the parliamentary sketch writers; look at the television duty log book.

This is one of those many under-acknowledged services women perform, a form of good works. Every morning on each newspaper, a picture editor lays out his treasure trove. Everyone sighs with relief when Nancy Dell'Olio, or Gwyneth Paltrow or Alexa Chung are out on the town. Pippa Middleton is a gift from God. The only thing to trump women and their dresses in newspapers is women without. Long Live Prince Harry's new girlfriend, the underwear model, Florence Brudenell-Bruce.

Sarah Sands is deputy editor of the London Evening Standard