Low key, middle key or high key - what is key?

So my second years are learning all about "key" when it comes to portraiture. They have been learning what it is and how to manipulate it with shutter speed and light control. Their first "key" assignment is that of "Low-Key" portraiture. This is a favoured area of many photographers when it comes to shooting. It allows us to create mood, drama and tension in our images and we even may consider converting them to "black and white"! Black and white with digital you say? You bet! Have a look and see for yourself.