Paste ciphertext to see which letters turn up most and how that compares to normal English.
English isn't evenly spread across the alphabet. E is about 12% of everything written. T, A, O, I and N follow. J, Q, X and Z barely appear. A cipher that swaps each letter for another, consistently, keeps that uneven shape. Only the labels change. Count the cipher letters, match the commonest to E, and you've started cracking it. Al-Kindi described the method in the ninth century, and it has broken substitution ciphers ever since.
The dark bars are your text. The orange tick in each column is how often that letter appears in ordinary English. Switch to “By count” to see the shape on its own.
Pick two letters from the text at random. The index of coincidence is the chance they're the same letter. English scores about 0.066. Perfectly random letters score 1/26, about 0.038. Because a substitution doesn't change the score, it tells you the cipher's family before you know anything else. William Friedman introduced it in 1922, and it's what our Vigenère solver uses to find the key length.
Pairs and triples show which letters sit together. The commonest English bigrams are TH, HE, IN, ER and AN. The commonest trigrams are THE, AND, ING and ION. If one three-letter group dominates your ciphertext, it's very likely THE, and that gives you three letters at once.
Counting how often each symbol appears in a ciphertext. In English, E, T, A, O and I are the most common letters. A simple substitution cipher changes the letters but not how often they appear, so the most common cipher letter is usually E.
In order: E, T, A, O, I, N, S, H, R, D, L, U. E alone is about 12% of all letters. The chart on this page uses the same English figures our solvers score against.
Common pairs and triples point to common words and endings. TH, HE and IN are the top English bigrams, and THE and AND the top trigrams, so the most common cipher trigram is often THE.
If no letter stands out, the cipher probably uses more than one alphabet, as Vigenère does. Frequency analysis on its own won't break that, but the Vigenère tool can.