What type of fitting is used to connect lengths of pipe in a straight line?

A coupling is designed specifically to connect two lengths of pipe in a straight line. This fitting allows for a continuous flow through the piping system without any change in direction, making it essential in applications where a straightforward connection is required. Couplings can be used with various types of pipes and materials, ensuring a tight seal to prevent leaks.

In contrast, an elbow fitting is used to change the direction of the flow, typically at a 90-degree or 45-degree angle, which does not serve the purpose of joining two pipes in a straight alignment. A T-fitting, or tee, is designed to connect three sections of pipe, allowing for branching off in different directions, which is also not applicable for straight connections. A union fitting operates similarly to a coupling but is specifically designed for ease of disassembly, often used in applications where pipes need to occasionally be taken apart.

Thus, the coupling is the appropriate choice for achieving a direct connection between lengths of pipe.