eat if I had food); the imperative is a mood to express a command, commonly a one word phrase – "¡Di!", "¡Talk!". Verbs express T-V distinction by using different persons for formal and informal addresses. (For a detailed overview of verbs, see Spanish verbs and Spanish irregular verbs.) Spanish syntax is considered right-branching, meaning that subordinate or modifying constituents tend to be placed after head words. The language uses prepositions (rather than postpositions or inflection of nouns for case), and usually—though not always—places adjectives after nouns, as do most other Romance languages. Spanish is classified as a subject–verb–object language; however, as in most Romance languages, constituent order is highly variable and governed mainly by topicalization and focus rather than by syntax. It is a "pro-drop", or "null-subject" language—that is, it allows the deletion of subject pronouns when they are pragmatically unnecessary. Spanish is described as a "verb-framed" language, meaning that the direction of motion is expressed in the verb while the mode of locomotion is expressed adverbially (e.g. subir corriendo or salir volando; the respective English equivalents of these examples—'to run up' and 'to fly out'—show that English is, by contrast, "satellite-framed", with mode of locomotion expressed in the verb and direction in an adverbial modifier). Phonology The Spanish phonological system evolved from that of Vulgar Latin. Its development exhibits some traits in common with other Western Romance languages, others with the neighboring Hispanic varieties—especially Leonese and Aragonese—as well as other features unique to Spanish. Spanish is alone among its immediate neighbors in having undergone frequent aspiration and eventual loss of the Latin initial sound (e.g. Cast. vs. Leon. and Arag. ). The Latin initial consonant sequences , , and in Spanish typically merge as (originally pronounced ), while in Aragonese they are preserved in most dialects, and in Leonese they present a variety of outcomes, including , , and . Where Latin had before a vowel (e.g. ) or the ending , (e.g. ), Old Spanish produced , that in Modern Spanish became the velar fricative (, ), whereas neighboring languages have the palatal lateral (e.g. Portuguese , ; Catalan , ). Segmental phonology The Spanish phonemic inventory consists of five vowel phonemes (, , , , ) and 17 to 19 consonant phonemes (the exact number depending on the dialect). The main allophonic variation among vowels is the reduction of the high vowels and to glides— and respectively—when