The folder size of A4 paper is usually 3 1.5cm and 28.5cm, and the width of the back depends on its capacity, which is generally about 3.5cm ... A binder is a kind of stationery that can bind and disassemble paper conveniently and freely. After the cover and paper are pasted, it is a loose-leaf book. Usually a loose-leaf book without paper is also called a bound book. A loose-leaf book needs corresponding binding holes on the paper, and some specifications also derive special paper punching machines, so the binding is more reliable; The other kind of paper is completely clamped by pressure and friction, which is relatively easy to spread.