Steed's Brolly - the weapon
Brolly 2HomeBrolly 4
But the umbrella is not just a part of his fashion, much more it's a weapon for John Steed in some fights.
Inside the brolly is a hidden steel blade, a sword, or better said a saber as Steed points out in The Charmers.
"You know a saber is my weapon."
Steed also has other canes with a hidden blade (The Joker, The Thirteenth Hole).
But Steed used the umbrella sword as a weapon only twice during The Avengers: in the episode The Grandeur That Was Rome and in Homicide and Old Lace.

But it's so popular in The Avengers as it's part of the intro of the colored Emma Peel season.
It can also be seen in a scene of The Correct Way to Kill in the umbrella shop.
There are only two fights with the brolly sword in all episodes of The Avengers.
Steed is using a short saber in The Grandeur that was Rome, and a smaller blade in Homicide and Old Lace.
Another brolly sword of Steed in his grey umbrella can also be seen in a small scene in The Correct  Way to Kill.
Steed used two different cane sticks with hidden blades, one of them in Small Game for Big Hunters.
It's a flexible blade, and another blade appears in The Joker.
It looks like is was made from a cross bayonet.
The cane as seen in episode The Joker
Steed defending himself with an umbrella sword from the gaslight ghoul club in Fog.
The Thirteenth Hole he explained Mrs. Peel that he has a special cane.
"It's a sword stick; it's got a sword in it." But soon enough Steed has to realize during a fight that he took the "the wrong stick."
No wonder he was mistaken!
The cane looks exactly like that one from Small Game for Big Hunters.
And Denis from Steel umbrella spotted another cane in A Sense of History.
In Small Game for Big Hunters is Steed using one of his cane swords.
John Steed's Fashion
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Umbrella shop scene in
The Correct  Way to Kill.
Fight scene in series 6 episode
Homicide and Old Lace.

Trivia:
Steed used his brolly sword in several comics such as in the Diana comic, story eight from 1967.

The umbrella sword in the set of The Avengers in Colour - John Steed & Emma Peel Deluxe Talking Action Figures, Product, Enterprise Ltd., Year: 2005