Duo-Tang was a brand name for paper folders made of cardstock paper. They are used to bind multiple sheets of paper by bending embedded brass fasteners through the holes of the paper and folding them down to keep them in place. Some models have three teeth on the fastener, and some have six (two on the top, middle and bottom). They could be different colours such as blue, green, red, yellow and black. They were often used as an organization tool for multiple pages of one subject, chiefly by elementary school students (however, many students usually transition to binders as they enter secondary school).
All previous Duo-tang products were subsequently replaced by Oxford / Esselte products or otherwise discontinued.
In Canada, the word duotang is used to refer to any such style of folder regardless of the actual brand.
"Fun fact: tang in Latin means tongue or pointed metal tool. And because there are two of these tongues that splits to clamp the pages down, thus duo-tang." - Samantha Smith, Proud Canadian.