Usage of a the word "BUT"

["Parker, Cassandra M. (EXCH)" <CMPARKER -at- INTERMEDIA -dot- COM>]

Hi all,

A quick question, what is the proper usage of a comma before the word
"but"?  I have seen it used correctly and incorrectly equally.

One example is below and was used with and without the comma before the
word "but"

       A.  "This sub-directory is easily accessible by all employees,
             but can only be updated by the Administrator."

       B.  "This sub-directory is easily accessible by all employees
             but can only be updated by the Administrator."

Thanks in advance,

Cassie


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