In 1910, enterprising teenager Joyce Hall started a postcard printing business in Kansas City. After a time, his brother joined the company, which they called Hall Brothers. The company thrived for a while, but by 1915, postcard sales had slowed. Apparently people had more to say to one another than could fit on a postcard and some were concerned with privacy issues. So, the Hall boys decided to print on 6″ by 4″ card stock (that happens to be the size of a COVID Card), fold it in half, and sell each card with an envelope. This gave people privacy in their writing and a little more space, while still letting them off the hook of a full length letter.
This may (the Smithsonian says “arguably”) have been the beginning of the modern day greeting card industry. Northern Card company has a little more to say on the subject (not so curiously, they don’t mention Hall Brothers).
In 1928, Hall Brothers became Hallmark.