Its vowel height is open, which means the tongue is positioned as far as possible from the roof of the mouth.
Its vowel backness is front which means the tongue is positioned as far forward as possible in the mouth without creating a constriction that would be classified as a consonant. This subsumes central open vowels because the tongue does not have as much flexibility in positioning as it does for the close vowels; the difference between an open front vowel and an open back vowel is equal to the difference between a close front and a close mid vowel, or a close mid and a close back vowel.
Most languages have some form of an unrounded open vowel. For languages that only have a single low vowel, the symbol for this vowel <a> may be used because it is the only low vowel whose symbol is part of the basic Latin alphabet. Whenever marked as such, the vowel is closer to a central [ä] than to a front [a].
Carbonell, Joan F. & Joaquim Llisterri (1992), "Catalan", Journal of the International Phonetic Association22 (1-2): 53-56
Cruz-Ferreira, Madalena (1995), "European Portuguese", Journal of the International Phonetic Association25 (2): 90-94
Fougeron, Cecile & Caroline L Smith (1993), "Illustrations of the IPA:French", Journal of the International Phonetic Association23 (2): 73-76
Jassem, Wiktor (2003), "Polish", Journal of the International Phonetic Association33 (1): 103-107
Martínez-Celdrán, Eugenio; Ana Ma. Fernández-Planas & Josefina Carrera-Sabaté (2003), "Castilian Spanish", Journal of the International Phonetic Association33 (2): 255-259
Rogers, Derek & Luciana d'Arcangeli (2004), "Italian", Journal of the International Phonetic Association34 (1): 117-121
Thelwall, Robin (1990), "Illustrations of the IPA: Arabic", Journal of the International Phonetic Association20 (2): 37-41