“Pioneer Era” postcards were from 1893-1898. They were all government postcards. They had no pictures printed on them. If they had pictures on them, they were hand illustrated, not commercially printed.
“Private Mailing Card” era was from 1898-1901. The government gave private printers permission to print and sell postcards with the inscription “Private Mailing Card” (PMC).
Postcard “Undivided Back” era was from 1901-1907. These cards were not allowed to have a divided back. All correspondence had to be written on the front of the card.
“Divided Back” postcard era was from 1907-1914. The left side of the card was used for messages, the right for the address.
White Border postcard era was from 1915-1930’s. They were 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 sheets.
Postcards from 1930 to 1944 were printed on a linen texture paper. This was called the “Linen Era” of postcards. They also used brightly colored inks.
The Chrome era of postcards is from1945-now. They are known for high photo quality and true living color. They generally have a glossy coating.