Skill distribution curve (the zones)
Talk of green zone, red zone and most of all black zone ringing comes up regularly on the various ringing forums.
The terms are analogous to the difficulty rating of ski slopes and were introduced in the context of ringing by Simon Linford in part two of a series of articles published in the Ringing World in 2002/3 entitled "Ringing at the Top?".
The articles can be found using the links below:-