The kana are organized into rows by their initial consonant, and each row pairs that consonant with the five vowels in the order a, i, u, e, o. The vowel row itself, sometimes called the A-row, contains あ い う え お in hiragana and ア イ ウ エ オ in katakana, pronounced /a/, /i/, /u/, /e/, /o/ respectively. Every other row follows this pattern. The K-row is か き く け こ (ka, ki, ku, ke, ko), the S-row is さ し す せ そ, the T-row is た ち つ て と, and the N-row is な に ぬ ね の. Each of these rows has exactly five characters.
Several rows contain irregular pronunciations that learners must memorize. In the S-row, the "si" slot is pronounced "shi" instead, giving し for /shi/. Similarly, the T-row uses "chi" and "tsu" for what would otherwise be "ti" and "tu", so ち represents /chi/ and つ represents /tsu/. The H-row also has a special case: "fu" is written with the character ふ rather than a hypothetical "hu". These irregularities are simply historical leftovers, and they are part of the basic chart rather than exceptions added later.
Not every consonant pairs with all five vowels in modern Japanese. The Y-row has only three characters: や ゆ よ (ya, yu, yo), with the "yi" and "ye" slots left empty. The W-row is even smaller, containing just わ (wa) and the particle を (pronounced /o/, used only as a grammatical marker). The standalone mora "n", written ん in hiragana and ン in katakana, sits apart from any consonant row because it is the only kana that represents a pure consonant without a vowel. Two historical characters, ゐ (wi) and ゑ (we), still appear in very old texts but are essentially unused in modern writing.