PROGRAM demcommon
     INTEGER*4 a, b, c
     COMMON a, b, c
     PRINT "Start .."
     a = 17
     b = 3
     c = 44
     PRINT "Initial values"
     PRINT a," ", b," ", c
     CALL doit ( )
     PRINT "Final values"
     PRINT a," ", b," ", c
     PRINT "Ok finish "
     END

     SUBROUTINE doit ( )
     INTEGER*4 g, j, k
     COMMON g, j, k
     g = 12
     j = 9
     k = 25
     PRINT "Subroutine values"
     PRINT g," ", j," ", k
     RETURN
     END
\

Start ..
Initial values
17 3 44
Subroutine values
12 9 25
Final values
12 9 25
Ok finish

Option Explicit' a good habit - variables should always have explicit creation

Dim Integer a' I'm usable anywhere

MySub

Print a ' will give 7 because was set inside MySub
Print b ' will give 49 (a*a)
Print c,d ' causes and error because these variables are unknown outside of MySub


Sub Mysub
Dim Integer b' I'm also usable anywhere - a new "common" variable
Local Integer c' I only work inside MySub
Static Integer d' so do I but I remember my last value
a=7 ' setting a "common" variable
b=a*a ' working with "common" variables inside a Sub
End Sub