of vowel letters denoting identical sounds today. Modern Greek orthography remains true to the historical spellings in most of these cases. As a consequence, the spellings of words in Modern Greek are often not predictable from the pronunciation alone, while the reverse mapping, from spelling to pronunciation, is usually regular and predictable. The following vowel letters and digraphs are involved in the mergers: Modern Greek speakers typically use the same, modern symbol–sound mappings in reading Greek of all historical stages. In other countries, students of Ancient Greek may use a variety of conventional approximations of the historical sound system in pronouncing Ancient Greek. Digraphs and letter combinations Several letter combinations have special conventional sound values different from those of their single components. Among them are several digraphs of vowel letters that formerly represented diphthongs but are now monophthongized. In addition to the four mentioned above (, pronounced and , pronounced ), there is also , and , pronounced . The Ancient Greek diphthongs , and are pronounced , and in Modern Greek. In some environments, they are devoiced to , and respectively. The Modern Greek consonant combinations and stand for and (or and ) respectively; stands for and stands for . In addition, both in Ancient and Modern Greek, the letter , before another velar consonant, stands for the velar nasal ; thus and are pronounced like English . In analogy to and , is also used to stand for . There are also the combinations and . Diacritics In the polytonic orthography traditionally used for ancient Greek, the stressed vowel of each word carries one of three accent marks: either the acute accent (), the grave accent (), or the circumflex accent ( or ). These signs were originally designed to mark different forms of the phonological pitch accent in Ancient Greek. By the time their use became conventional and obligatory in Greek writing, in late antiquity, pitch accent was evolving into a single stress accent, and thus the three signs have not corresponded to a phonological distinction in actual speech ever since. In addition to the accent marks, every word-initial vowel must carry either of two so-called "breathing marks": the rough breathing (), marking an sound at the beginning of a word, or the smooth breathing (), marking its absence. The letter rho (ρ), although not a vowel, also carries rough breathing in a word-initial position. If a