To The Letter Meaning
(idiomatic) Literally, exactly, to follow the rules as they're written.
Example: You should follow what your boss told to the letter. Otherwise, you'll lose your job.
2011, Phil McNulty, Euro 2012: Montenegro 2-2 England 
Capello warned his players that caution was not an option as they went in search of the result that would take England to Euro 2012. And his message was carried out to the letter in the opening exchanges as England played with a tempo and threat Montenegro struggled to subdue.