vorletzt
penultimate
Describes the position directly before the last position in an order or sequence.
Die entscheidende Szene befindet sich im vorletzten Kapitel des Buches.
The decisive scene is in the penultimate chapter of the book.

penultimate / second-to-last
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penultimate
Describes the position directly before the last position in an order or sequence.
Die entscheidende Szene befindet sich im vorletzten Kapitel des Buches.
The decisive scene is in the penultimate chapter of the book.

The word is a clear compound of 'vor' (before) and 'letzt' (last). So it literally means 'before the last one.'
Wir nehmen den vorletzten Zug um 18:30 Uhr.
We are taking the penultimate train at 6:30 p.m.
'Vorletzt' (penultimate) is often used in reference to sequences in time or space. Typical combinations are 'der vorletzte Tag' (the penultimate day), 'die vorletzte Woche' (the week before last), 'das vorletzte Mal' (the second-to-last time), or 'der vorletzte Platz' (second-to-last place).
Das war das vorletzte Mal, dass ich ihn gesehen habe.
That was the second-to-last time I saw him.
Like other adjectives, 'vorletzt' (penultimate) can also be used as a noun to refer to a person or thing in this position. It is then capitalized and declined.
Der Vorletzte im Rennen war völlig erschöpft.
The second-to-last runner in the race was completely exhausted.
last
'letzt' and 'vorletzt' both describe a position at the end of a series. 'letzt' means the very last position; nothing comes after it. 'vorletzt' means the position directly before that. Learners can easily confuse the words because both have to do with the end. A simple guide is: with three parts, part three is the last part, and part two is the penultimate part.

Der vorletzte Bus ist schon abgefahren, aber der letzte kommt in zehn Minuten.
The second-to-last bus has already left, but the last one comes in ten minutes.
Im letzten Kapitel wird die ganze Geschichte erklärt.
The whole story is explained in the last chapter.