Bookbinding 101: Binder's Board
Binder's board is what puts the hard in a hard bound book. Also called book board, Davey board, gray board, and chip board—although chip board isn't usually what you actually want, because "chip board" usually refers to a lower quality product. Even so, "chip board" is sometimes used as a synonym for binder's board.
Some types of binder's board are smoother than others. Davey board is a higher standard of board and has a smoother finish than many other brands of binder's board.
Like paper, binder's board has a grain. If you buy large pieces, it's helpful to mark the grain direction across the whole piece in several places so that when you cut it down and have leftover scraps, you'll know the grain direction of those smaller pieces.
We have made some very large books that required extra thickness for the cover so we glued two pieces of book board together and pressed it overnight. Too large for any press, we placed a wooden board on top, and on top of this board we placed heavy stuff—whatever we could find. When gluing the two book board pieces together, we made sure to cross the grains, so that one board's grain went one way and the other board's grain ran the other. This helped the cover board to not warp after gluing them together.
For box making, smaller books, and some rounded spines, we like to use a thinner book board that's .059" thickness (or about 1.5mm or 1/17"). For most of our other books, we like using thicker book board as it give our books a bit more heft. Also, with larger books, the thicker board lends more support and sturdiness to the book. The thicker board is .098" thickness (about 2.5mm or 1/10" thick). For books in the range of about 4"x5" (10.16 x 12.7 cm) to about 8"x10" (20.32 x 25.4 cm) we like .080" thickness of book board (about 2mm or 1/12").
Binder's board also comes in black and white. These are especially useful when using a thinner paper for your book's cover or end sheets if you do not want the typical grey binder's board to show through, but instead prefer either a black or white background behind the thin paper.
Black or white board can also be handy when making board books, the kind of books for toddlers with the heavy duty, no tear pages, which typically have white book board as the core of each page. Usually, board books have rounded corners, perhaps to make them safer for kids, but also because the rounded corner is more durable—if the book had a ninety degree corner, a toddler would smoosh it out of shape in no time. The rounded corner takes the wear and tear better. When making your own books, if you happen to drop a cover piece you are working on and it squishes just a bit, don't despair. Try using your bone folder to shape it back into place. If that doesn't work, consider turning the book's corners into rounded corners.