In an ideal world, a bed iron is installed as shown in figure 1.

Important points to note:
The downside of the ideal installation is that in most applications the bed iron, and box spring, shows below the rail. To avoid this problem, many people opt for this alternate installation:

Notes on this alternate method (figure 2):
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Horton Brasses Inc., 49 Nooks Hill Road, Cromwell CT 06416, 800-754-9127