BASH scripting

[Michael Havens]

just starting with this so please, bear with me....

Anyways I am looking at this beginner's script:

   #! /bin/bash
   myvar=0
   while [ $myvar -ne 20 ]
   do
           echo $myvar
           myvar=$(( $myvar + 1 ))
   done

Now my mind can wrap itself around everything in this script except for tis
line:

   myvar=$(( $myvar + 1 ))

Where my difficulty arises is why the '$' before the '(('?
(it was geast fun running that script with a minus sign instead!)
:-)~MIKE~(-:
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