MySQL Error #1071 – Specified key was too long; max key length is 767 bytes

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When I executed the following command:

ALTER TABLE `mytable` ADD UNIQUE (
`column1` ,
`column2`
);

I got this error message:

#1071 - Specified key was too long; max key length is 767 bytes

Information about column1 and column2:

column1 varchar(20) utf8_general_ci
column2  varchar(500) utf8_general_ci

I think varchar(20) only requires 21 bytes while varchar(500) only requires 501 bytes. So the total bytes are 522, less than 767. So why did I get the error message?

#1071 - Specified key was too long; max key length is 767 bytes

37 s
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