Arabic belongs to the group of Semitic alphabetical scripts in which mainly the consonants are represented in writing, while the markings of vowels is optional. The Arabic alphabet has 28 letters and is read from right to left, top to bottom.
As the following picture illustrates, the shape of these letters changes depending on their position in the word; whether isolated, in the beginning of the word, in the middle, or at the end. Letters whose names appear in blue are those which can't be joined on the left side; letters with names in red change shape according to position in the word, while other letters are not changed in all cases.
Several letters in the Arabic alphabet share the same shape, and are differentiated only by the number and placement of dots on the letters. Of the basic 18 shapes, 2 are used for three letters, 6 are used for two letters, and the remaining 10 are used for one letter each.