ASME B16.9, ASME B16.28 and, MSS SP43 are the key specifications covering the dimensions and the manufacturing tolerances of butt weld fittings. ASME B16.25 covers the preparation of butt weld connections between pipes and fittings. ASME B16.49 covers the marking details.
Among all of the fittings, elbow is the one most often used.
Elbow is used when a pipe changes direction, they can turn up, turn down, turn left, right, or any angle in between.