The data layer contains the page index for the Thomas Brother Map Guide, which references a page number in the paper map book published by Thomas Brothers. This grid, copyrighted by Thomas Brothers, is commonly used to reference the general location of many geographic data types, both in Thomas Brothers and in the real estate, as well as other, fields.
This dataset has been enhanced by LA County CEO Urban Research to include hypothetical page boundaries where Thomas Brothers does not produce map pages, but are still within the county.