In the 20th century, newer designs emerged that were shorter and predominately made of stitched or braided leather, with a flexible spring inside the handle. The slight flexibility and resilience of the handle gave these small clubs a whip-like action. Law enforcement sources from the mid-20th century preferred to divide these into two categories: "Blackjacks", which have a mostly cylindrical striking head, and "saps" which have a flat, usually oval-shaped head. In common usage, these terms have become interchangeable, so a "sap" of this kind is sometimes more precisely called a "flat sap, slap jack" or "beavertail sap" to differentiate it. The sap's flat profile makes it easier to carry in a pocket and spreads its impact out over a broader area, making it less likely to break bone. However, it can also be used to strike with the edge for more focused impact, though this was discouraged by most police departments for precisely this reason. There are several variants of these weapons that use different materials, such as steel instead of lead for the weight, or plastic for the covering. Some variants use powdered metal or even sand for the weight inside the head, usually called a "soft sap," which reduces the likelihood of bone fractures.