pairwise(iterable) yields (s0, s1), (s1, s2), (s2, s3), ... until the source has fewer than two items left. It works on any iterable (list, tuple, generator, file object), not just sequences, and it does not rebuild internal state between iterations. Use it whenever a calculation reads two adjacent items at a time: deltas, ratios, monotonic checks, edge construction from a path. It costs O(n) time and O(1) extra memory regardless of input size.

Before 3.10, the standard recipe was a 4-line wrapper around itertools.tee. tee(iterable) returns two independent iterators sharing the same source, then advancing one by next(b, None) makes it lead by exactly one item. zip then pairs them up and stops at the shorter iterator (the trailing one). It is worth knowing this recipe both for older codebases and because the same tee + offset + zip shape generalizes to triple-wise or N-wise sliding windows.

Pairwise turns 'compare each item to its predecessor' into a flat loop with no index bookkeeping. Tracking the best monotonic streak becomes a five-line function: bump run when the relation holds, reset it otherwise. The all(a <= b for a, b in pairwise(seq)) idiom is the cleanest way to ask 'is this sorted?' without sorting it first. The same trick generalizes to 'are all consecutive deltas equal?' for arithmetic-progression detection.