In a tee fitting labeled 4x2-1/2, what does the first number indicate?

In a tee fitting labeled 4x2-1/2, the first number indicates the diameter of the straight-through opening. This means that the fitting has a primary passage or opening that has a diameter of 4 inches, which is the main line through which the fluid or gas will flow. Understanding the labeling of fittings is crucial in plumbing because it allows the installer to know how to properly connect pipes of different sizes.

The second number, in this case, 2-1/2 inches, represents the diameter of the side opening, which allows for a branch connection. Recognizing the correct dimensions is essential for ensuring compatibility with other components of the plumbing system and maintaining proper flow rates. The other options relate to different characteristics of fittings that do not pertain to diameter identification.