What is the difference between a Jacke and a Mantel?
Are there any cases in which one could be both?
Are they properly defined or are they elastic concepts?
They are clearly defined:
Jacke is jacket, so just a normal jacket, nothing special about it.
Mantel is cloak or coat, so actually longer than a normal jacket.
When it comes to length, Mantel goes down to the knee or even more.
They are both fairly well-defined, separate concepts:
Jacke - jacket (light, shorter outerwear)
Mantel - coat (heavy, longer outerwear)
A Jacke is appropriate for mild, autumnal weather and usually is waist-length or shorter. A Mantel is appropriate for cold, winter weather and is usually ankle or knee-length.
This same concept also applies to derivative nouns, such that:
Regenjacke - A (light, shorter) rain jacket
Regenmantel - A (heavier, longer) raincoat
Also note that Mantel also means cloak, sheath, or covering and is used in a variety of contexts such as for electrical sheaths on wires, or for ceremonial cloaks, etc.
Wikipedia Entry (German): Mantel
"Der Mantel ist ein der Jacke ähnliches Kleidungsstück, der wesentliche Unterschied besteht in der Länge. Die Differenzierung, ab welcher Länge die Jacke aufhört und der Mantel anfängt, ist von der Epoche und der Mode abhängig."
Duden Entry: Mantel
"längeres Kleidungsstück mit langen Ärmeln, das [zum Schutz gegen die Witterung] über der sonstigen Kleidung getragen wird"
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