As requested by Diann MacRae:
A very clear and concise essay explaining the H-P system of plumage
classification is presented on the Ontario Field Ornithologists web page.
www.interlog.com/~ofo/home.html
It can also by found through Dan Victor's "Birding in Cascadia" page.
I found this essay extremely interesting and a valuable aid in
shorebird aging.
Hope this helps,
Brian Scott
Vancouver, BC