The long vowel is a sound made by lengthening the short vowel without changing its quality. For example, ā (あー） is pronounced as ā, i.e. by lengthening the vowel [a], not as two sounds of a, such as a a ( ああ ). The ‘ー’ marks the long vowel, and it is important to maintain sufficient duration when pronouncing long vowels. Japanese native speakers perceived long vowels as having the same duration as two moras.
In katakana ( カタカナ ( 片仮名 )), the long vowel is denoted by ‘ー’. On the other hand, in hiragana ( ひらがな ( 平仮名 )), the long vowel is noted by a, if it comes after sounds of the a-row, and i and u if it precedes sounds of the i and u-row sounds respectively. For long vowels following the e-row sounds, they can be either noted by e or i. Similarly, long vowels following the o-row sounds can be noted by either o or u. Do be careful as there are cases when the pronunciation does not correspond to its kana orthography. For instance, ou (おう) is pronounced as ō (おー), not o and u separately. The same applies to ei ( えい ); it is pronounced as ē (えー).