How do you choose what sort of "key" or legend to use on your map? (If any!)

[lostatsea]

I am taking a different view on your question than others I think ! A key or legend is generally a side note to explain the information on the map that might not have obvious meanings. Ex. The difference between the city and town and temple symbols. Or in atlas style explain the symbol for waterfall or restroom or camp sights. If your map is tactical it will explain that the color green stands for the Orc occupied areas, blue Human occupied. Etc. Key or legend is usually placed out side the main areas of the map to be easily and clearly read and not interfere with the map itself. I once asked if there was a traditional (specific) placement Ex upper right / lower left corner. Seems that depends mostly on the shape of the map and where it will be out of the way. If you are doing a real world map that might vary by area mapped time period being replicated. etc. So my take is; If you have symbols you need a key. Place where it fits in the best and try to keep it in style with the rest of your map !

[Jalyha]

As for the other stuff, I would follow some obvious rules: If you feel the map informations aren't obvious ( complicated icons, high number of icons, specific locations), add a key to explain them. If you need to add some additional informations that can't be depicted on your map, you might want to add some informations box, some bars or cartouches to detail it.

So my take is; If you have symbols you need a key. Place where it fits in the best and try to keep it in style with the rest of your map !

That's what I was looking for. So the difference, really, is determined by what information you're including?