Beaufort cipher
Vigenere with the subtraction the other way round, which makes it its own inverse.
What it does
Where Vigenere adds the key letter to the plaintext letter, Beaufort subtracts the plaintext letter from the key letter.
That small change has a practical consequence: the operation becomes its own inverse. Run ciphertext through with the same keyword and the plaintext comes back. No second table, no separate procedure, and one fewer thing for a tired operator to get wrong at three in the morning.
Where it came from
Attributed to Sir Francis Beaufort, the naval officer better known for the wind scale. The same principle was later built into the Hagelin M-209, a mechanical cipher machine used in quantity by American forces during the Second World War.
How strong is it
Exactly as strong as Vigenere, which is to say not very. The keyword still repeats, and a repeating key is what Kasiski examination attacks. Find the period and the message separates into simple shift ciphers.
Do not confuse it with the variant Beaufort, which subtracts the key from the plaintext instead. That one is not self-inverse and is genuinely a different cipher.
Related
Autokey removes the repeating key entirely and is meaningfully stronger than either.