Plural of 'circular': printed or written notices, advertisements, or informational letters that are distributed to many people.
From 'circular' (an adjective used as a noun) plus the regular plural '-s.' In this sense, 'circular' became a noun meaning 'a circular letter,' which became common in business from the 1800s onward.
Circulars were the spam of the 19th century—they arrived by post instead of email, people complained about junk mail just as much as today, and savvy advertisers were already trying to make them stand out with better design.
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