It all comes down to the HB graphite grading scale, which is used to categorize the graphite core of the pencil. The graphite core's hardness is commonly written on the pencil—look for a number (such as "2", "2-1/2", or "3"), and the higher the number, the harder the writing core and the lighter the imprint produced on the paper.
Number 2 pencils are most commonly made from grades 2 and 3 wood, while numbers 2-1/2 and 3 are typically made from grade 4 and 5 wood, respectively. This makes sense if you think about it: If you were to write with a number 2 pencil, you'd want something that was hard enough to do a job but not so hard that it would be difficult to erase. A grade 2 pencil will usually have a softwood core and a lead weight between 150 and 250 mg, while a grade 3 has a harder core and a heavier lead weight of approximately 300 to 400 mg.
Numbers up to 10 can be made from either grades 2 or 3 wood or both. Numbers 11 and 12 are only made from grade 4 wood, and 13 and up are only made from grade 5 wood. This is because as you go up in size, the amount of lead needed increases dramatically, so only larger cores are capable of holding such heavy loads. Also, as you go up in size, the quality of wood required to make the pencil decreases.
Let's take a closer look. The position of a pencil on the HB graphite grading scale is determined by the hardness of its graphite core. A hard lead will leave an indentation that is very close to being a circle; a soft lead will not even begin to fill in the hole you make with it.
A standard #2 pencil has a medium-hard lead that produces a hollow cone 10 millimeters in diameter at the top of your column of ink. A #4 pencil has a hard lead that leaves an indentation 6 millimeters deep; a #6 leads sink into the wood 3 millimeters deep.
A #2 pencil can be used to draw simple lines, while a #5 must be used when greater detail is required. A #00 must be used to mark holes when screwing things together.
The term "grade" refers to the size of the lead used to make the pencil. There are several grades of lead available, from the softest (#2) to the hardest (#0). Each grade has numbers indicating their hardness. A lower number means a softer lead, while a higher number means a harder one.
In addition to grade, there are also shades of brown or black lead, which determine the color of the lead itself.
It has to do with the "hardness" or "softness" of the graphite substance that is used to mark the paper. The greater the number, the harder and lighter the pencil markings left. A number 2 pencil is softer and blacker than a number 3 or 4 pencil.
The hardness of the lead inside the pencil can be changed by adding different types of wood or plastic to the inner case. This changes the weight of the pencil and also its writing quality. For example, a number 2 pencil might have polyvinyl acetate (PVA) plastic in the casing while a number 4 would use wood. Both types of pencil share the same ferrule at the other end which keeps the leads together on one piece of paper.
When you write with a pencil, carbon from the lead is transferred to the surface it is pressed against, leaving a clear line where the lead was pressed down. The type of lead used affects how much force is required to write with each stroke of the pen. Harder leads require more effort because enough pressure must be applied to the point of the pen to leave a visible mark on the page. Soft leads leave less of a mark because they require more pressure to make a complete circle around the tip of the pen.
Number grades of pencils are based on the hardness of their leads.
Today, however, most HB pencils are labeled with a number such as 2B, 4B, or 2H to denote the degree of hardness. A 4B, for example, is softer than a 2B, whereas a 3H is tougher than an H. Before the 1950s, there were only two types of pencils: 2B and 4B.
As far back as 1821, Charles Frederick Gunter invented the first successful hand-held mechanical pencil. It was called the "Gunter's Scale" because it included both hard and soft points on a single stick. The first known use of the term "hardness" when referring to pencils is in an advertisement in the September 1913 issue of Popular Science Monthly. At that time, the ad stated that "A No. 2 pencil can be used instead of a No. 5 ball point pen."
The first known use of the term "HB" as a designation for a type of pencil comes from an ad in the October 1920 issue of School Arts Magazine. At that time, the ad described these new pencils as having "a hard black lead wrapped around a soft white wood core."
By the 1930s, more than one manufacturer was making HB pencils. Each company may have been putting their own twist on the design, so today we would call them unique styles rather than specific brands.