Mechanical pencils come in two main types. The 'clutch' pencil has a gripping mechanism which is released by pushing the end button. This allows the lead to slide freely out of the pencil. Releasing the button grips the lead in its new position.
Clutch pencils tend to be used by artists, designers and engineers for drawing, rather than writing.
Propelling pencils allow the lead to be extend a small amount each time the mechanism is operated. The lead is never free to slide out, as with the clutch pencil. Some propelling pencils have a twist mechanism and others a push mechanism. Propelling pencils often have a guide tube to support the fine lead and reduce breakage and on some models this tube can be retracted when not in use to protect it from damage. Propelling pencils are mainly available with 0.5mm and 0.7mm leads, the 0.7 being a little more robust and less likely to break.
Our large range of mechanical pencils can be found here.