Madd [Prolongation] means an extension of the duration of vowels.
The Madd Letters are: [ ا ] whatever the diacritical mark of the preceding consonant, [ و ] with the preceding consonant having [ ُ ] and [ ي ] with the preceding consonant having [ ِ ].
These letters are pronounced with Madd when they precede a Hamzah in the same word [Connected Madd], or in the next word [Detached Madd].
The duration of Madd is five counts [Harakah]. In detached Madd the duration may be only two counts [Harakah].
Examine the examples to know how these letters are written in the `Uthmanic calligraphy of the Qur"an.