With photographic composition I often hear people talk about 'leaving enough headroom' for portrait photographs.

1. What is headroom?
2. How can I properly apply it to my photographs?
3. (for @mattdm) What are the psychological and art history underpinnings of this 'rule?'

With photographic composition I often hear people talk about 'leaving enough headroom' for portrait photographs.

1. What is headroom?
2. How can I properly apply it to my photographs?
3. What are the psychological and art history underpinnings of this 'rule?'