Pencils are generally graded using the 'Graphite Scale(石墨含量)' which (measure) how soft or hard the lead is. Graphite is a naturally soft substance and clay is mixed with it to make it harder, (mean) a lead with a high graphite content is known as a soft lead and pencils with a soft lead will make a dark mark when used. (contrary), a lead with less graphite and a higher clay content is known as a hard lead and will make a lighter mark.
Soft leads are graded using the letter B' (indicate) how 'black' the mark, they make is. Numbers are then used to mark the degree of (soft). For example, a 2B lead is softer than a B lead and will produce a blacker mark.
Hard leads are graded in the same way, use an ' H' to show how 'hard' they are. A 2H is harder than an H and a 6H is really hard. HB (hard black) leads (consider) to be the happy medium on the graphite scale. There are no specific industry (standard) that set down how black HB leads should be, so an HB pencil made by one producer will not necessarily leave the same mark an HB pencil from a different producer.
