Low Key Portraits
Pete Bobb's 1st Class Photography
‘I personally guarantee that you will be absolutely delighted with your photographs
or I won’t charge you a cent
and you get to keep all of your prints.’
Several definitions are in current use for ‘Low Key’ portraits.
Common definitions include ideas such as ‘An average of 2 stops darker than 18% grey’ and other similar definitions that mystify non-specialists.  
‘Low Key’ portraits have more dark tones than mid or light tones, leading the eye to seek out and linger on the few light tones to be found in the portrait.
The whites of the eyes are powerful 'magnets' for our attention in this style of portrait, leading to a 'study of ' or 'focus on' the face.

For simplicity's sake, I consider any portrait with a black backdrop as being ‘Low Key’.
Dark clothing or hair continues the ‘Low Key’ concept.
The Master Painters of the 17th Century (such as Rembrandt) created many low key portraits with good effect.