What is the difference between thick and thin pages?

Only the page itself. Thick pages are 1.2 mm: a sturdy build with a substrate between each page, and noticeably more heft in the hand. Thin pages are 0.5 mm: two pages set back to back, no substrate, a much slimmer book at the same spread count. Both are lay-flat, and the swatch box includes both so you can feel the difference.