Way back in ancient history there were mostly illiterate people. Books were expensive. So the vast majority of people not only didn't have books but they wouldn't be able to use one if they did.
Then, in the 1500s in England and Europe, things began to change. Mass printing was developed - and at the same time more and more people learned how to read. So now industrious printers made up cheap little "tiny books" with songs in them. People liked songs and they liked to know the words of songs. Sort of like how kids today go to YouTube to learn lyrics of songs. So these were chapbooks. As more and more people learned how to read, and as printing got better organized, these tiny books went out of fashion.
Fast forward to modern days. The term chapbook has one main use now - for a short collection of poetry, around 40-60 pages. It's not a huge tome of poetry, but it's not just one or two poems either. It's a reasonable collection that someone can read through and enjoy without being so large that it's called a book. Chapbook is a fairly arbitrary term, but it's sort of like a novella. It's enough poems to satisfy you, with the thought that there will be other chapbooks coming along perhaps on other themes or for other time spans.
If you're a poet, it's well worth it to consider compiling a chapbook!
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