What kind of construction is "eine Tasse Kaffee"?

Question

I was a little surprised to not find more information about this kind of construction in German, e.g.

eine Tasse Kaffee

ein Glas Wasser

My intuition said that this could be a genitive (in the second noun, I mean) that goes unnoticed because of the absence of an article — seems like it's mistaken.

Another plausible explanation I found in some sites (like this one) is that it is a partitive (which in German would actually be a double-nominative construction), but I couldn't find any authoritative source — e.g. the Wikipedia article says that the partitive is 'unknown in German'.

So, what is it?

Answer 1

Eine Tasse Kaffee

is a cup of coffee in English. Where most languaguages use a partitive genetive German has a special construction nominative + nominative. The first noun describes the quantity a cup, a bucket, a sack, the second noun describes of what: coffee, water, flour.

Today the construction for how much of what is nominative + nominative. But it used to be nominative + genitive:

ein Becher Weines (Schiller, around 1800)

As feminine nouns have no genitive ending, it was ein Glas Milch and it was clear from the meaning of the words that the first noun described how much and the second noun of what.

So one learned that the genitive ending of the second noun was not necessary and in the course of time, it was dropped.

If you can find nothing about this problem in your grammar book you should try to get a better one. You can check the quality of a grammar book just by checking what it says about a special problem such as this special construction with a double nominative.

P.S: Don't consider Wikipedia as an authority in languages. They compile a lot of things and often they don't get it right. As you see German had a partitive genitive (for nouns of masculine and neuter gender), but the genitive ending was dropped. But there are a lot of expressions where a partitive relation is expressed with von: tausende von Büchern/tausende Bücher. Or:

Vieles von dem, was in Wikipedia über Grammatikbegriffe gesagt wird, kann man nur als verunglückt bezeichnen.

Answer 2

A short way of answering it would be: interpret it as a quantifier. And quantifiers go without flective cases (i.e. they go with nominative case that has no special suffix):

ein Dutzend Äpfel

eine Hundertschaft Polizisten

ein Herde Schafe

eine Anzahl Kinder

eine Menge Unruhe

ein Stück Kuchen

ein Glas Milch

eine Prise Salz

eine Mütze Schlaf

fünf Scheffel Weizen

20 Sack Zement

ein Sack Flöhe

sieben Glas Bier

acht Gläser Bier

neun Flaschen Wein

fünf Dachdeckergesellen

drei Chinesen mit dem Kontrabass

zehn kleine Afrikanerlein

viel Lärm um nichts

kaum Erquickliches

eine Wagenladung alte Jungfern

massig Zoff

That's just so in German. Other languages have other conventions. In Russian, for example, you would usually have to use genitive (пять стаканов пива). Not so in German.