Audio barrel connectors are a style of connector most commonly used for connecting audio equipment such as headphones and small microphones. They are characterized by a cylindrical plug divided into two or more sections along its length and having an indented region near one end that is engaged by a spring loaded contact in a mating jack to provide retention force. Several standard (nominal) sizes are available, the most common being the 1/8" (3.5 mm) format commonly used for audio connections in desktop and mobile devices, and the older 1/4" (6.35 mm) size more commonly found on older or non-portable equipment where durability is of generally greater interest than minimal size.