improving the electrical contact and preventing the pin from falling out. The other end of the plug has a lug connector to which a length of flexible insulated equipment wire can be attached, which is either screwed, soldered, or crimped into place. On banana plugs for laboratory use, an insulating plastic cover is usually fitted over this rear end of the connector, while banana plugs for speaker connections often use uninsulated metal bodies. The rear end of a 4 mm plug often has a 4 mm hole drilled in it, either transversely or axially, or both, to accept the pin of another 4 mm plug. This type is called a stackable or stacking plug. For high voltage use, sheathed versions of the banana plug and socket are used. These add an insulating sheath around both the male and female connectors to avoid accidental contact. The sheathed male plug will not work with an unsheathed female socket, but an unsheathed male plug will fit a sheathed female socket. Individual banana plugs and jacks are commonly color-coded red and black, but are available in a wide variety of colors. Dual banana plugs are often black with some physical feature such as a molded ridge or thick tab, marked "Gnd" indicating the relative polarity of the two plugs. Besides plugging into specific banana jacks, banana plugs may plug into "five-way" or "universal" binding posts. Derived plugs The PL-259 plug is widely used as an HF cable connector; it is a shielded form of a 4 mm banana plug. A number of widely used plugs are based on combining two or more banana plugs in a common plastic housing and other features for ease of use and to prevent accidental insertion in other such plugs. Many of these plugs are derived from the "double banana" plug consisting simply of two banana plugs spaced apart (measured from center to center of each individual plug). The inch spacing originated on General Radio test equipment during the 1920s, and their type 274-M dual-plug is a notable example from that era. The housing may allow the connection of individual wires, a permanently attached coaxial cable providing both signal and ground, or a coaxial connector such as the BNC connector shown in the photo. Conventionally the tab located on one side of the dual banana plug denotes the ground, shell, or negative (reference) side of the connector. Miniature banana