When are noun abbreviations written in capitals?

Question

Some noun abbreviations seem to be written completely in capitals, while others only begin with a capital.

• Abb. (Abbildung)
• Bhf. (Bahnhof)
• Jh. (Jahrhundert)
• EL (Esslöffel)
• AKW (Atomkraftwerk)
• PLZ (Postleitzahl)

At first it seemed to be related to composite words, but that rule wouldn't cover Jahrhundert.

When are noun abbreviations written in capitals?

Answer 1

Duden has a couple of rules devoted to that topic.

There don't seem to be any hard and fast rules but if you are willing to accept exceptions, it boils down to the following (already partly mentioned by you):

• initialisms are written in caps (AKW - for the three words Atom, Kraft and Werk that create Atomkraftwerk)
• shortened words are not (Abb., Bhf., etc.)

I don't know how many exceptions to a rule you need in order to say that rule doesn't exist in the first place. Although I'm not going to count there certainly seem to exist some exceptions for this one, e.g.

• Pkw which would fall into the first category but Duden recommends lower-case
• GmbH which is camel-cased (as are a lot of other law-related terms; check out this list of German acts)

---

In addition to that, there is another category: Kurzwörter. They usually are in the common vocabulary and sometimes aren't known to be not the "real word". Still they are abbreviations and therefore relevant to the current question. They are always lower-case which strengthens their appearance as common words.

• Trafo (Transformator)
• Tacho (Tachometer)
• Lok (Lokomotive)
• Uni (Universität)

---

Duden also has a little article on the topic.

Answer 2

UPDATE

It seems impossible to give a general rule but some patterns can clearly be made out:

A) If the abbreviation of the noun is marked by a point, only the first letter has a capital.

Jh. -> Jahrhundert
Abb. -> Abbildung
Jgg. -> Jahrgänge

The reason for this seems to be that the point takes the function of marking the abbreviation, so that it is not necessary to use capitals inside the word to mark it. In the sentence

Im vierten Jh. vor Christus

"Jh." is no valid word so that the point must serve as abbreviation; thus you don´t need to use "JH."

But f.ex. in some technical languages, this can be different.

Dies steht in BGBl. 17 vom 23.11.2010.

B) If no point is used, capitals inside the abbreviation can be used to mark it clearly.

It seems to depend on the position of the letter that is used in the abbreviation, in the abbreviated word:

1) Letters which mark the beginning of a discrete part of the noun, such as

a) a sub-noun/-adjective

AKW -> Atomkraftwerk

Usually, these letters are capitals, but sometimes they are not:

• letters not at the beginning of the word in some abbreviations

  Pkw -> Personenkraftwagen

  Hbf -> Hauptbahnhof

• letters used to abbreviate some units

  km -> Kilometer

  ml -> Milliliter

But:

EL -> Esslöffel

The question why this is so, is very interesting and I must admit I have no sweeping idea yet. The exceptions apply to very well known, frequently used nouns and the abbreviations are clearly marked as such. Perhaps an abbreviation such as "el" is not clear enough from the user's point of view.

b) a syllable

Here, the same is true.

VO -> Verordnung (Ver-ordnung)

If the letter used in the abbreviation is none of these, it is usually no capital:

Na -> Natrium
MHz -> Megahertz

Answer 3

There is no rule for that, as canoo.net tells us:

Für die Groß- und Kleinschreibung von allein stehenden Abkürzungen gibt es keine festen Regeln.

So you have to memorize every single one, sorry.