Pencils are available in different grades, with letter and number codes to identify how hard or soft the lead is. A ‘normal’ pencil is HB, or in the American system, a #2.
In the UK, and (I think) most of Europe, pencils are always labelled with one scale – H for Hard, or B for soft, with a number to say how hard or soft. HB is the middle of the range, and by far the most common type. For sketching, though, a softer lead is usually preferred, often 2B or even 4B. For more technical drawing or very light lines, a harder lead works better, like a 2H. The scale goes up to 9 at each end – 9B to 9H, with the extreme ends of the scale being a bit too extreme for most uses.
Sometimes, you’ll also find an F pencil – Firm – between the HB and the H (the 1 is missed off).
The US usually follows the UK system for drawing pencils, but for office use commonly refers to HB as #2.
#1 is B, #2 is HB, #2 1/2 is F, #3 is H, and #4 is 2H.
This is why the UK system is used for drawing pencils – the US system doesn’t cover the sort of range used for sketching. I usually draw with 4B lead – well off the scale.
- Article on Pencil Hardness on Pencil Pages.