In work involving lead came (the metal piece between the glass in traditional windows) the lead knife does the cutting and is used for assembly. It resembles a hooked blade linoleum knife, has a wooden handle usually and often has a lead weight attached to the end of the handle. The blade is very thin and sharpened on the outside as opposed the a linoleum knife which is sharpened on the inside. The weight of the knife allows the craftsman to cut the lead came by wiggling the knife back and forth on the came, the weight takes it through the lead without crushing the profile of the came. The heavy end can be used to tap the glass into place and won't crack the glass as the lead of the knife is relatively soft. A high quality tool makes any crafts activity a joy, a good tool is an extension of your hand.