sentences:  Postmiles on callboxes are approximate due to a convention that all callboxes on the northbound or eastbound side of a divided roadway are assigned even numbers while all those on the southbound or westbound side are assigned odd numbers, even though the call boxes are often located directly across from one another.Alphabetic prefixes on postmile markers and bridges differ from callbox prefixes because the callbox system is maintained by each county, while Caltrans maintains postmile markers and bridge signs.The following table lists postmile and callbox prefixes by county.