A hose coupling is a connector on the end of a hose to connect (or couple) it with another hose or with a tap or a hose appliance, such as an irrigation sprinkler. It is usually made of steel, brass, stainless steel, aluminium or plastic. Most often, this size of coupling is associated with high water volume needed for direct attach on structure fires. Other uses include a devise to place large volume of water at the entry or base of a fire where an appliance (ie. gated valve) will be attached in order to run two separate 45 mm hoses off of it. This way, fire can be attacked at two separate angles.
You will notice markings on the male coupling (thread visible on the exterior) and female coupling (thread is visible inside). These markings are called Higbee Cuts and are often used for lining up the threads. This makes it a lot easier to connect a new length of hose.
Another important note is that becoming disoriented and lost inside a structure fire is a danger every Firefighter faces when entering an unfamiliar building that is filled with smoke. One technique we use is … find the hose and if we come across the female couple first, that is the direction out.