What is a Concertina Sketchbook?

Concertina sketchbooks are gaining in popularity due to their format expanding like a fan versus a regular pad or book which displays two pages at a time. They can be made from reconstituted folders, books or purchased blank. I love teaching this class where we use inks, acrylic, pastels, watercolor, made or found papers, markers, bric-a-brac and other elements that create an unfolding story across numerous pages.