# Why I can get a key from frequency in Caesar Cipher?

I took a cryptanalysis class in university and I learned Caesar Cipher. But I cannot understand why frequency gives me the "key".

In a meterial, the following sentences are explained:

When Eve tabulates the frequency of letters in this ciphertext, she gets: I=14, V=13, S=12, and so on. The most common character is I with 14 occurrences. This means key = 4.

What I cannot understand is the last sentence, "This means key = 4". Most frequency alphabet is "E" which has 12.7 frequency and the number of "E" is 4 I know. But how can I compute it from the frequency? I want to totally understand this mechanism.

A B C D E F G H I J ...

The Caesar cipher starts with the letter and goes through the alphabet using the key, wrapping around after 'Z'. So if 'I' is the most common letter then this is probably the enciphered letter 'E'. And since index('I') - index('E') = 8 - 4 = 4, that's the key that was used.