Like Steve said, they're for alternative layouts for use generally with engineered lumber, which in residential construction typically means trusses.
Typical layout of 16" gives you 6 X 16" = 8 ft.
Alternate layout of 19 3/16" gives you 5 X 19 3/16 = 8 ft.
Here's more:http://www.carpentersrule.com/sys-tmpl/door/
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