After all these necessary preliminaries, we get back to the question of how to determine the quality and length of a vowel from the written form of the word. We remember from the section on the vowel system that there are mainly three kinds of vowels in German:
long vowels, which are typically close; more precisely vowels that are long if stressed and short if unstressed, and close unless written 〈ä〉.
short vowels, which are always open; more precisely vowels that are short and open irrespective of stress.
Schwa, that is, the vowel written 〈e〉 in some unstressed syllables where it is pronounced [ə].
(Note that close unstressed vowels are contained in the first group although they are in fact short.)
Below, rules are given for determining which of these three cases applies when, which is often, albeit not always, possible from the written form of the word, provided the stressed syllable is known. The scopes of the rules are not always mutually exclusive; then the earlier rules take precedence.
The vowel in a light prefix or suffix (cf. section on stress) is short and open. If it is an 〈e〉, it is pronounced
as a short open [ɛ] in a prefix when the prefix ends with a consonant: her- (only when unstressed), ent-, er-, ver-, zer-,
as a short open [ɛ] in a suffix when an 〈r〉 follows, and
as a Schwa [ə] in all other cases.
The standard makes much more distinctions about which alternate pronunciations are admissible in which contexts, but the above rules cover all cases in a standard-conformant way.
All vowels are long when there is an 〈h〉 following in the same syllable. The vowels 〈a〉, 〈e〉, and 〈o〉 are long when they are geminated. The vowel 〈i〉 is long when it is written 〈ie〉 or 〈ieh〉. Vowels can be long without being marked as long by these means; typically, only stressed vowels are in this way marked as long.
Note that most intervocalic 〈h〉 within morphemes are not pronounced but serve only to indicate the length of the preceding vowel, e.g. nahe [nɑːə], Mühe [myːə]; nonetheless the hyphenation rules demand that the 〈h〉 is written on the new line with the following syllable. Exceptions are Ahorn amd Uhu and interjections like aha: there the 〈h〉 is spoken and belongs to the second syllable.
All vowels are short when the following consonant is geminated; when 〈k〉 and 〈z〉 are geminated they become 〈ck〉 and 〈tz〉. Mainly stressed vowels are in this way marked as short, plus vowels other than 〈e〉 in light suffixes when they lose their final position: -in–>-innen, -nis–>-nisse, although not -ig–>*-igge. With very few exceptions, geminated consonants appear only at the word end or between vowels, or when additional consonants have been inserted by an inflexion, e.g. es hallte from the verb hallen.
Consonants written as digraphs or trigraphs (〈ng〉, 〈ch〉, 〈sch〉) cannot be geminated, nor can 〈x〉. In addition, 〈st〉 and 〈tsch〉 also behave like single consonants that cannot be geminated. The vowel before 〈ng〉 and 〈x〉 is always short, for 〈ch〉, 〈sch〉, 〈st〉, and 〈tsch〉 see rule 7 below.
〈s〉 can in principle be geminated to become 〈ss〉 but due to changing usage of 〈ß〉 and 〈ss〉 in different countries at different times one has to be cautious about inferring the length of the preceding vowel:
In German and Austrian texts adhering to the rules of the spelling reform in the 1990ies, 〈ss〉 marks the preceding vowel as short and 〈ß〉 marks the preceding vowel as long.
In German and Austrian texts written prior to that reform or ignoring its rulings, the same holds with the exception that the vowel preceding an 〈ß〉 can be either short or long when no vowel follows.
In Swiss texts, 〈ß〉 is not used and the vowel preceding an 〈ss〉 can be either short or long.
In a polysyllabic word, a vowel is long if it is followed (before the next vowel or the word end) by at most one consonant that could be geminated but is not. In particular, the stressed French and Latin endings (cf. section on stress) contain long vowels unless they end in two consonants. The endings -ik as in Musik and -it as in Bandit are pronounced short as [-ɪk] and [-ɪt] in wide parts of Germany; however, the standard insists on the Northern variant with long vowel [-iːk] and [-iːt] (again with an exception: in chemistry and mineralogy Sulfid [zʊlˈfiːt] and Sulfit [zʊlˈfɪt] must be discernible).
The vowel in uninflected monosyllabic words such as articles, pronouns, and prepositions is
long when there is a single 〈r〉 after the vowel at the word end, in dem, wem, den, wen, and, restricted to North German usage, also in schon,
long when the vowel is at the end of the word, and
short in all other cases.
As many of these words are unstressed, thus short, in the context of a sentence, “long” means basically “close”. As already mentioned, the vowel of her [heːr] changes to open when it appears unstressed in a composition as her- [hɛr-].
The vowel in inflected monosyllabic words such as adjectives and nouns is
long when inflected forms have also a long vowel by rule 4 (Tal–>Täler, Weg–>Wege, schwül–>schwüler) or when there is a single 〈r〉 after the vowel at the word end, and
short in the remaining cases.
Nearly all examples with short vowels are of English origin (non-English are Bus, Chef, Kap, and Tic). Nouns build their plural mostly by appending 〈s〉, and other derivations are done by geminating the final consonant, e.g. poppig from Pop. In order to reduce these cases, the spelling reform in the 1990ies modified the former Mop, Step, Stop, and Tip to become Mopp, Stepp, Stopp, and Tipp, but they missed out on Bit, Bob, Bus, Chat, Chef, Chip, Clip, Club, Dip, fit, Flip, Flop, hip, Hit, Jet, Job, Kap, Kit, Lop, Mob, Net, Pep, Pop, Set, Shop, Slip, Snob, Spot, Sprit, Strip, Tic, Top, Trip, Twen, and Web.
The length of a vowel that is followed by 〈ch〉, 〈sch〉, 〈st〉, or 〈tsch〉 at the word end or between vowels is not always predictable. The same holds for the vowel followed by 〈ß〉 or 〈ss〉 in that position in those cases where rule 3 yields no result. The following guidelines are helpful for guessing the length of the vowel with quite some reliability:
〈e〉 and 〈i〉 before any of these consonant patterns are short.
All vowels before 〈sch〉 are short, with very few exceptions where 〈u〉 und 〈ü〉 are long before 〈sch〉.
〈u〉 und 〈ü〉 are mostly long before 〈ch〉, with the exception of nouns directly derived from verbs not already containing the 〈u〉, e.g. short Bruch, Geruch, but long Suche.
〈a〉 and 〈ä〉 are mostly short before 〈st〉.
In the remaining cases, the length of the vowel is hardly predictable, perhaps with a slightly better chance of being short.
Before 〈r〉 and a following dental sound ([d], [t], [s], [ʃ]), 〈a〉, 〈ä〉, 〈e〉, and 〈ö〉 can be short or long. Northern pronunciation often prefers long vowels where Southern prefers short ones, e.g. in erst and Behörde; in other cases, one of short or long is used nearly everywhere, e.g. Herz [hɛrts] and Erz [ʔeːrts]. The standard has again a preference for Northern pronunciation, although it recognises some short variants as regionally valid Austrian or Swiss pronunciations. The actual distribution of long and short pronunciation for these words is in fact much more irregularly scattered across the German-speaking countries, the above North-South distinction only being a general trend.
A vowel is long if the vowel in the word stem is long by the preceding rules, even when it is followed by two or more consonants, e.g. malst [mɑːlst] from malen [ˈmɑːlən]. In some words, the stem is not easy to recognise: nebst (from neben), stets (from stet), beredt (from reden).
In nearly all remaining cases where a vowel is followed by two or more consonants, it is short. There are, however, a handful of exceptions to this rule: Obst, Jagd, Magd, Krebs, and Papst had two syllables until not long ago, Mond acquired its final -d only recently, and Keks and Koks are recent loans from English cake and coke.