Pioneer Postcards - 1870-1898 (these are the first ones!)

Private Mailing Cards 1898-1901 (these are what postcards were called at this time)

Also, postcards are often dated by whether they had a divided or undivided backs (the line down the middle)

Undivided back - 1901-1907

Divided back - 1907-1915 (and to present, of course)

"Pre-Linens". Indicates any view card that predates linen cards. Usually they're artificially colored. There are tons of these around, usually pre-1930.

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I'm not sure if any of this history is relevant to casino cards, because of the dates involved.