Before the internet and digital tools became intrinsic of everyday botany, scientists used tools like this punched-card key to identify Colorado mosses.
Created by William Weber, former curator of the University of Colorado Museum Herbarium, the key requires its user to select cards corresponding to particular traits of the moss in question. Whether it be the shape of the leaf, the color of the plant, or the elevation from which it was observed, each trait has a matching numerical code. After stacking the selected cards the user is left with a single hole that still shows light, which codes to a particular genus. This way anyone with a general knowledge of botany can identify the genus to which a moss belongs.
These days, keys for identifying plants can be found online, like this one from the Southwestern Environmental Information Network (SEINet). Check out the rest of SEINet for a treasure trove of botanical tools, such as a map search function, plant lists, and images of specimens housed in museum collections around the world.