How many pages does a book leaf have?

Question

A co-worker asked a riddle that got me wondering about the anatomy of books and book binding methods.

The Riddle:

A leaf is torn from a paperback novel. The sum of the remaining page numbers is 15,000. What pages were torn out?

According to the answer (and a lot of the internet), it claimed that a leaf:

is a single sheet bound in a book, and a leaf has two pages.

I thought this assumption was vague and that a leaf can either have two pages or four pages depending on how it is bound. This assumption also changes the answer of the riddle.

Leaf with two page binding:

Leaf with four page binding:

Are both definitions of a book leaf correct? Or is there only one? Is one more commonly assumed than the other?

Answer 1

A quick Google search revealed that

Very generally, "leaves" refers to the pages of a book, as in the common phrase, "loose-leaf pages."

A leaf is a single sheet bound in a book, and a leaf has two pages. (Source: Biblio)

A folded grouping of leaves is a "gathering", so the left image under "leaf with 4 pages" in your question is actually a gathering of 4 leaves (8 pages). The right hand image is a gathering of 2 leaves (4 pages)

Note that if you tear one leaf out of a bound book, as books are more commonly bound in gatherings, often one other leaf will also become loose, and maybe fall out — especially if the leaf torn out is in the centre of a gathering. But that is a separate leaf, not the other half of the same.