(Little insider info: I have never used all of them....for the drawing below only 3 were scribbled with (6B,2b & 2H))
The grading goes all the way from 9B up to 9H and bang in the middle is the F...
This well researched article by the team at Wonderstreet takes an in-depth look at all things pencil.
(...)In this article, we explore the world of pencils and all of the different variations on this popular artistic medium. This guide has been written as an introduction for beginners and as a reference for more experienced artists who may be searching for a new product or looking to compare their current favourite to other pencils on the market.
Historically, pencils have been used since the 16th century, when a large deposit of pure graphite was discovered in the UK. Unfortunately, it was also discovered that graphite could be used to line the moulds for cannonballs, so the mines where the graphite was excavated were seized by the British Crown and graphite for pencils had to be smuggled out. Later methods for producing pencils included reconstituting graphite powder and mixing pure graphite with clay. This process is still in use today.
A rating system was put in place during the 20th century and is commonly used around the world to denote the hardness or softness, otherwise known as “blackness” of a pencil. The rating 9H is the hardest pencil, which produces a very faint grey value. The rating HB is the most commonly used for writing utensils, and 9B is the softest.
A second ranking system, ranging from #1 to #4, is also in use. This system was developed by the famous manufacturer of Conté crayons and adopted in the US by John Thoreau, father of poet Henry David Thoreau.(...)