Ba is the second letter of the Arabic script in the modern order and the second in the older abjad order, where it carries the numeral value two. Its basic shape is a shallow horizontal bowl with a single dot placed beneath it. That bowl is the most productive skeleton in the script: the same stroke with two dots above is ta, with three dots above is tha, with one dot above is nun, and with two dots below is ya. Five of the twenty-eight letters share one outline and are told apart by their pointing alone.
Ba joins on both sides and changes shape with position. Standing alone, or at the end of a word, it keeps the full bowl and its rising tail. At the beginning of a word and in the middle it contracts to a short upright tooth carrying its dot, so a reader meets it as a very different mark depending on where it falls. Almost every Arabic letter behaves this way, which is why an alphabet chart showing only isolated forms tells a reader rather little about the running script.
Ba is a moon letter, one of the fourteen before which the lam of the definite article is pronounced rather than assimilated. Its name descends from the Northwest Semitic beth, meaning house, and the letter is the direct cognate of Hebrew bet, Syriac beth and Samaritan bit. As throughout the Arabic sequence, the inherited name kept its shape and lost its sense: no Arabic speaker hears a house in it.