What do the initials HB on a pencil stand for?

A pencil marked "HB" is hard and black; a pencil marked "HH" is very hard, and a pencil marked "HHBBB" is very hard and really, really black!

http://pencils.com/hb-graphite-grading-scale/

By the beginning of the twentieth century, a combination letter-number system had been established and was in use by nearly all European pencil makers, and was also used for some American-made pencils. This system is still in use today, and provides for a wide range of grades, usually consisting of the series:

9H, 8H, ... , 2H, H, F, HB, B, 2B, ... , 8B, 9B

where 9H is the hardest, 9B is the softest. At the same time, a number-only system was in use, particulary in the U.S., which is still in use. The table below indicates approximate equivalents between the two systems:

USA English
#1 B
#2 HB
#2½ F
#3 H
#4 2H

http://www.pencilpages.com/articles/simmons.htm

Or...
http://www.thealmightyguru.com/Pointless/PencilGrades.html

A good book to check would be "The Pencil: a History of Design and Circumstance" by Henry Petrowski. As I recall, he discusses the meaning of the pencil markings What-A-Mess mentioned, but nobody has any good ideas on what some of them mean, for example "F." Doesn't seem to mean anything important in either German (a lot of the German pencil makers were German. Ebehard Faber comes to mind) or English!