# EQUIVALENCE and COMMON Interaction

A common block can extend beyond its original boundaries if variables or arrays are associated with entities stored in the common block. However, a common block can only extend beyond its last element; the extended portion cannot precede the first element in the block.

## Examples

The following two figures demonstrate valid and invalid extensions of the common block, respectively.

A Valid Extension of a Common Block
An Invalid Extension of a Common Block

The second example is invalid because the extended portion, B(1), precedes the first element of the common block.

The following example shows a valid EQUIVALENCE statement and an invalid EQUIVALENCE statement in the context of a common block.

      COMMON A, B, C
      DIMENSION D(3)
      EQUIVALENCE(B, D(1))      ! Valid, because common block is extended
                                !   from the end.

      COMMON A, B, C
      DIMENSION D(3)
      EQUIVALENCE(B, D(3))      ! Invalid, because D(1) would extend common
                                !   block to precede A's location.

Parent topic: Effects of Equivalency and Interaction with COMMON Statements