Slack bytes (unused character positions) are added before each synchronized item in the records. For Example -

01 STUDENT.
   05 STUDENT-NO       PIC 9(02).
   05 STUDENT-NAME     PIC X(12).
   05 STUDENT-GRADE    PIC 9(02).
   05 STUDENT-CLASS    PIC X(03).

After the compiler inserts the slack bytes, the declaration is modified. For instance, after STUDENT-NO, the next item STUDENT-NAME starts from the boundary. Similarly, for STUDENT-GRADER, the STUDENT-CLASS starts from the boundary after the insertion of slack bytes. The declaration after slack bytes is -

01 STUDENT.
   05 STUDENT-NO       PIC 9(02).
  [05 SLACK-BYTES      PIC XX.  INSERTED BY COMPILER]
   05 STUDENT-NAME     PIC X(12).
   05 STUDENT-GRADE    PIC 9(02).
  [05 SLACK-BYTES      PIC XX.  INSERTED BY COMPILER]
   05 STUDENT-CLASS    PIC X(03).

The below diagram represents the memory allocation along with slack bytes -

Four slack bytes are inserted if we do not use the SYNC clause. The variables (followed by slack bytes) should declare with SYNC clause to avoid the slack bytes as shown below -

01 STUDENT.
   05 STUDENT-NO       PIC 9(02).
   05 STUDENT-NAME     PIC X(12) SYNC.
   05 STUDENT-GRADE    PIC 9(02).
   05 STUDENT-CLASS    PIC X(03) SYNC.

The diagram below represents the same allocation after introducing the SYNC clause. There will be no slack bytes anymore -

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