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A structure chart is a hierarchy chart 
  that shows 
    the data 
    and control 
  information flows between the modules. 
  (Figure 63.1 shows a partial structure chart.) 
  
Each module 
  is represented as a rectangle. 
Each data flow (or data couple) 
  is shown as an arrow 
    with an open circle at the origin end. 
A control couple (a flow of control information such as a flag or a switch setting) 
  is shown as an arrow 
  with a solid circle at the origin end, 
see the control couple labeled Reorder flag between Process sale and Process transaction in Figure 63.1. 
(Note: In this program design, 
  Initiate reorder is an independent (not shown) level-2 module 
    called by Process transaction when the Reorder flag is set.) 

As appropriate, 
  the names of 
    the data elements, 
    data composites, and/or 
    control fields 
  are written alongside the arrows.
  
A structure chart 
  does not show 
    the program’s 
      sequence, 
      selection, or 
      repetitive logical 
    structures; 
  those details 
    are inside the modules, 
      which 
    are viewed as black boxes. 
    
  However, 
    some designers 
      identify 
        high-level case structures 
      by adding 
        a transaction center 
      to 
        a key control module. 
  For example, 
    the solid diamond 
      (the transaction center symbol) 
      at the bottom of 
        the Process transaction module 
      indicates that, 
        based on the transaction type, 
          either 
            Process sale, 
            Process customer return, or 
            Process shipment arrival 
        is called.


A data couple 
  might list 
    a composite item; 
      for example, 
        Get data 
          passes 
            a complete transaction 
              and 
            the associated master record 
          back to 
            Process transaction. 
Higher-level modules 
  generally select 
    substructures 
      or
    specific data elements 
  from 
    a composite 
    and 
  pass them down to 
    their children. 
    
At the bottom of the structure, 
  the detailed computational modules 
    accept 
      and 
    return 
      data elements.

The structured program designer’s objective 
  is to define 
    independent, 
    cohesive, 
    loosely 
    coupled 
  modules. 
  
Coupling is 
  a function of 
    the amount of 
      data and control information 
        flowing between two modules, 
  and 
    the structure chart 
      graphically shows 
        the data and control flows. 
        
An excessive number of 
  data or control flows 
    suggests 
      a poor design 
        or 
      a need for further decomposition.