Onion skin was thinner, yellowish and more transparent than vellum, so you could tape a piece over top of a drawing and create an alternative layout, or study, without revising the original drawing.

When we started using CAD, I remember one of the old timers taping a piece of it over the CAD monitor so he could make a sketch and explain what he wanted me to change. I laughed then, but now I am the one who can't keep up with technology.