The Atharvaveda and Rgveda Prātiśākhyas also give the number of svars as 13 ; those that commonly appear in the Vedic and the Purānic are included. This means that besides the above 5 there are lengthened forms of the first four only viz. ā, ī, ū, ŕ (Aa¸ [- ¸ } ¸ ?R ) and not that of l ( ; ). This brings the number to 9.
Now â combines with î to give ( A + [ ) as e ( e ) ; â combines with ī to give ( A + [- ) as ai ( eo ) ; â combines with û to give ( A + ] ) as ô ( Aao ) ; â combines with ū to give ( A + } ) as au ( AaO )– these vowels are called mixed or diphthongs, mishrit. Like the four lengthened forms these four diphthongs are also of double prosody, that is they have two mātrās.
The Solar system and the Sanskrit Alphabet
There is a shlöka in the Aitareya Ārānyaka II.2.4 which says  :-
vyañjanaih+eva rātri+ira+pruvanti svaraih+ahāni .
By the consonants the earth-nights are delineated. By the vowels the days.
The svars are the shining energies of the Sun. The consonants are the reflected energies of the moon. If â ( A ), as the symbolic Brahman, the Unknown, is suppressed among the 13 vowels then we are left with 12, the number of the Sun-signs.