Write the text in a zigzag over several rails and read it rail by rail; the zigzag is drawn out.
Procedure
How it works
The rail fence is a transposition: the letters stay the same, only their order changes. Write the message diagonally down and up over $r$ rails, then read each rail from left to right. With 3 rails, WE ARE DISCOVERED RUN AT ONCE becomes WECRUOERDSOEERNTNEAIVDAC.
The zigzag repeats every $2(r - 1)$ characters. An offset starts the text somewhere inside the first period. To decrypt, count how many characters fall on each rail, cut the ciphertext into pieces of these lengths and follow the zigzag again.
The key is just the number of rails (and the offset), so all keys can be tried by hand. The letter frequencies are those of the plaintext, which immediately shows that a transposition was used.
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