Reference: Legend or Key
The image below is taken from a trench map with a more extensive legend than most.
WWI Marginal Information: Legend or Key
The area labelled as "Reference" in the bottom margin of a trench map is what is more commonly referred to as a "Legend" or "Key". Trench map legends were usually not a complete listing of all the symbols used and some maps contained no legend at all. For this reason you may wish to consult the web page on this site called symbols used on the maps for a more comprehensive listing.
