Interleaving alternates between two lists, taking one line from each in turn: first line of A, first of B, second of A, second of B.
What it produces
A single list where related items sit adjacent. If the two inputs are aligned — the same subject in the same position — the output pairs them.
Where pairing matters
Old URLs and new URLs for a redirect map. Titles and their descriptions. Keywords and their target pages. Anywhere two parallel lists describe the same set of things.
Alignment is a precondition
Interleaving assumes position n in A corresponds to position n in B. If either list has a gap, everything after it pairs incorrectly.
Try it: Interleave Two Lists on SeoWolf's Notepad.