Black iron cross fitting can be used for a variety of applications, such as plumbing, industrial ventilation, hanging and supporting equipment, or routing wiring and cables. Some specific common uses include:

Connecting black iron pipes for natural gas or propane piping systems.

The cross fittings allow pipes to intersect at 90 degree angles.

Connecting ductwork in industrial ventilation and exhaust systems.

The cross fittings join sections of sheet metal ducts.

Anchoring equipment such as heaters, pumps or compressors.

The cross fittings provide sturdy connection points to mount and hang heavy equipment.

Routing electrical conduit and wiring.

The open cross section allows wires and cables to pass through at intersection points.

Constructing shelving and racking systems.

The cross fittings can join vertical and horizontal pipes to form the frame for shelving units.

Building fence frames and gates.

The cross fittings connect sections of black iron pipe to form the structural support for fences and gates.

Fabricating table and cart bases.

The cross fittings attach legs made of pipe to the underside of a table top or cart platform.

Assembling scaffolding structures.

Cross fittings are commonly used to join sections of pipe to construct scaffolding frames and platforms.

So black iron cross fittings have many structural applications where pipes need to be joined together at right angles for purposes such as plumbing, ventilation, equipment support, electrical routing and light-duty framing systems. They provide a simple but sturdy means of connecting black iron pipes in industrial settings.