How the shift works
Each letter moves a set number of places along the alphabet and wraps round at the end. With the usual shift of 3, A becomes D, B becomes E, X becomes A and Z becomes C, so HELLO turns into KHOOR. Capitals stay capitals, and numbers, spaces and punctuation are left as they are. The Shift box takes 1 to 25; a shift of 26 would bring every letter back to itself.
Cracking a Caesar code
Paste the code into the Code box, click in the Shift box and press the up arrow: the plain text is redrawn at every step, and with only 25 shifts the right one shows up within seconds. A one-letter word in the code is a good clue, since in English it is nearly always A or I. A shift of 13 is ROT13.
Questions
How do I undo a shift to the left?
A shift of 3 to the left is the same as a shift of 23 to the right, so put 23 in the Shift box. Any left shift of n is a right shift of 26 minus n.
Why is it called Caesar?
The Roman historian Suetonius wrote that Julius Caesar used it in private letters with a shift of three, which is why 3 is the usual shift and the one this page starts on.
Is it safe for real secrets?
No. With 25 possible shifts anyone can try them all by hand. For something private, use Encrypt Text.