a few cases, such as ad 'name' (dative ada), the underlying form is retained in the spelling (cf. at 'horse', dative ata). Other exceptions are od 'fire' vs. ot 'herb', sac 'sheet metal', saç 'hair'. Most loanwords, such as kitap above, are spelled as pronounced, but a few such as hac 'hajj', şad 'happy', and yad 'strange' or 'stranger' also show their underlying forms. Native nouns of two or more syllables that end in /k/ in dictionary form are nearly all //ğ// in underlying form. However, most verbs and monosyllabic nouns are underlyingly //k//. Vowels The vowels of the Turkish language are, in their alphabetical order, , , , , , , , . The Turkish vowel system can be considered as being three-dimensional, where vowels are characterised by how and where they are articulated focusing on three key features: front and back, rounded and unrounded and vowel height. Vowels are classified [±back], [±round] and [±high]. The only diphthongs in the language are found in loanwords and may be categorised as falling diphthongs usually analyzed as a sequence of /j/ and a vowel. Vowel harmony The principle of vowel harmony, which permeates Turkish word-formation and suffixation, is due to the natural human tendency towards economy of muscular effort. This principle is expressed in Turkish through three rules: If the first vowel of a word is a back vowel, any subsequent vowel is also a back vowel; if the first is a front vowel, any subsequent vowel is also a front vowel. If the first vowel is unrounded, so too are subsequent vowels. If the first vowel is rounded, subsequent vowels are either rounded and close or unrounded and open. The second and third rules minimize muscular effort during speech. More specifically, they are related to the phenomenon of labial assimilation: if the lips are rounded (a process that requires muscular effort) for the first vowel they may stay rounded for subsequent vowels. If they are unrounded for the first vowel, the speaker does not make the additional muscular effort to round them subsequently. Grammatical affixes have "a chameleon-like quality", and obey one of the following patterns of vowel harmony: twofold (-e/-a): the locative case suffix, for example, is -de after front vowels and -da after back vowels. The notation -de² is a convenient shorthand for this pattern. fourfold (-i/-ı/-ü/-u): the genitive case suffix, for example, is -in or -ın after