Used when emphasizing that nothing has been excluded from a group or collection. I donated all my books, down to the last one.
Pro Tip 2/3
Idiomatic Emphasis
Adds emphasis to the totality of what's being discussed, often in a dramatic or significant way. She listened to his every word, down to the last sigh.
Pro Tip 3/3
Literary Flair
The phrase can add a touch of drama or literary quality to a sentence. The antique collection was astonishing, down to the last stamp.