Prototype plate letters on prototype freight cars

[ATSF5700BOB]

 How did the plate "c", plate "f" designations have their beginnings? I know that if a certain freight car exceeds certain height/width restrictions, then that car has a notice attached that says: exceeds plate c or exceeds plate f. Also how many different plate clearances are there? For example was there a plate "A', etcetera?
          Thanks
   ATSF5700Bob

[BillD53A]

A draftsman calls a drawing a 'plate'.
The drawings give the relationship between car length, and width, and truck centers;  all of which govern the amount of overhang on curves.  Railroad employees know which curves have clearance restrictions....which are 'plate B', 'plate C', etc.  They need to know if a car will clear any obstructions on a curve, or if it exceeds the clearances.   This info, pertaining to curve clearances, is contained in employee timetables.
The actual 'Plate' drawings can be found in an 'Official Railway Equipment Register' from your time period.   You might pick up an old copy at a train show, etc.

[Dr EMD]

see this site

http://www.trainweb.org/utahrails/drgw/plate.html

[ATSF5700BOB]

Thank you Dr EMD for the link to the plate site. I found it to be very informative. Also thank you BillD53A for your response on how the plate definition started and the beginnings of plate clearances.
                                 ATSF5700Bob