PE = Pioneer Era [1893-1898]
Government postcards with no pictures. Any pictures were hand illustrated, not commercially printed.
PM = Private Mailing card [1898-1901]
Government gave permission to private printers to print and sell postcards with the inscription “private mailing card”.
UB = Undivided back [1901-1907]
All correspondence had to be written on the front of the card, not being allowed to have a divided back.
DB = Divided back [1907-1914]
Left side was for messages, the right side for addresses.
WB = White Border [1915-1930s]
Usually printed in tinted half-tones partially for style, partially due to ink shortages at the end of WWI. The white border gave more leeway when cutting down the postcard sheet.
LE = Linen Era [1930-1944]
Printed on a linen texture paper, using brightly colored inks.
CE = Chrome Era [1945 – current]
They are known for high photo quality and true living color. They generally have a glossy coating.