text: In some types of phonetic transcription (e.g. pronunciation respelling), "long" vowel letters may be marked with a macron; for example, ⟨ā⟩ may be used to represent the IPA sound /eɪ/. This is sometimes used in dictionaries, most notably in Merriam-Webster (see Pronunciation respelling for English for more).
  Similarly, the short vowel letters are rarely represented in teaching reading of English in the classroom by the symbols ă, ĕ, ĭ, ŏ, o͝o, and ŭ. The long vowels are more often represented by a horizontal line above the vowel: ā, ē, ī, ō, o͞o, and ū.
  Origin
  Vowel length may often be traced to assimilation. In Australian English, the second element  of a diphthong  has assimilated to the preceding vowel, giving the pronunciation of bared as , creating a contrast with the short vowel in bed .
  Another common source is the vocalization of a consonant such as the voiced velar fricative  or voiced palatal fricative or even an approximant, as the English 'r'. A historically-important example is the laryngeal theory, which states that long vowels in the Indo-European languages were formed from short vowels, followed by any one of the several "laryngeal" sounds of Proto-Indo-European (conventionally written h1, h2 and h3). When a laryngeal sound followed a vowel, it was later lost in most Indo-European languages, and the preceding vowel became long. However, Proto-Indo-European had long vowels of other origins as well, usually as the result of older sound changes, such as Szemerényi's law and Stang's law.