- Pipe is designated by Trade Size, aka nominal diameter (roughly according to its Inside Diameter). Pipe is not normally Flexible.
Pipe is designated by trade size, and generally has an inside diameter somewhere near its nominal size.
For instance, a standard 3/4-inch schedule 40 iron pipe (whether its made of iron, steel, brass,etc.) has an O.D. of about 1.05 inches (nowhere near 3/4 inch), and an I.D. of .742 inches - very nearly 3/4 inch.
To handle more pressure, pipe "grows" inwardly. A given size of pipe must always be able to thread into its mating appliance. Therefore its O.D. must remain constant as its wall thickness is varied.
For a more thorough discussion on the various types of PIPE, see About PIPE.