Gurmukhi is the name of the script used in writing primarily Punjabi and, secondarily, Sindhi language. The word gurmukhi seems to have gained currency from the use of these letters to record the sayings coming from the much (lit. mouth or lips) of the (Sikh) Gurus. The letters no doubt existed before the time of Guru Angad (even of Guru Nanak) as they had their origin in the Brahmi, but the origin of the script is attributed to Guru Angad. He not only modified and rearranged certain letters but also shaped them into a script. He gave new shape and new order to the alphabet and made it precise and accurate. He fixed one letter for each of the Punjabi phonemes; use of vowel-symbols was made obligatory, the letters meant for conjuncts were not adopted and only those letters were retained which depicted sounds of the then spoken language . There was some rearrangement of the letters also.
It is commonly accepted that Gurmukht is a member of the Brahmi family. Brahml is an Aryan script which was developed by the Aryans and adapted to local needs. According to an opinion, the Brahmi script was introduced between the 8th and the 6th centnries BC. It does not concern us here whether the script was foreign or local, but it has now heen established, on the basis of internal evidence, that whatever be its name, the Aryans did have a system of writing which must have been borrowed freely from local scripts. The Iranians ruled in the Punjab in the 3rd and 4th centuries BC. They brought with them Aramaic script, which helped in the growth of Kharosthi largely used in the Punjab, Gandhar and Sindh between 300 BC and 3rd century AD. But even then Brahmi, which in its development in the Punjab had undergone several changes, was commonly used along with Kharosthi . There are coins of the Bactrian kings and inscriptions of the Kushan rulers having both scripts on them. Brahmi was, of course, more popular on account of its simple curves alternated with straight strokes. Hence, in due course, it replaced Kharosthi and became the single script with composite features effected by various local and neighbourly influences. With the growth of literary and cultural activity during the Gupta period (4th and 5th century AD), the Brahmi script improved further and became more expansive and common.
Immediately later, it developed, especially in northern India, fine curves and embellished flourishes with a small headline over each letter, and became rather ornamental. This stage of Indian script was called Kutil, meaning curved. From Kutil evolved the Siddhamatrika which had the widest use in northern India. Some scholars think that these two scripts existed simultaneously. From the sixth century to the ninth, Siddhamatrika had a very wide use from Kashmir to Varanasi. With the rise of regional languages taking the place of Sanskrit and Prakrit, regional scripts grew in numnber. Ardhanagari (west), Sharda (Kashmir) and Nagari (beyond Delhi) came into use, and later both Sharda and Devanagari, an offshort of Nagari, started their inroads into the land of the five rivers. This is evident from the coins of the Ghaznavids and Goris minted at Lahore and Delhi. It is also known that the common (non-Brahman and non-official) people used a number of scripts for their temporal and commercial requirements. Of these Lande and Takre characters were most prevalent.It is on account of these currents that scholars have tried to establish relationships of Gurmukhi with Devanagri (G.H. Ojha), Ardhanagan (C.B. Singh), Siddhamatrika (Pritam Singh), Sharda Diringer) and Brahmi (generally) . Some ascribe it to lande and some others to Takn, a branch of Sharda used in Chamba and Kaligra. The fact is that it is dervied from or at least allied to all these and others mentioned above in their historical perspective. Regionally and contemporarily compared, Gurmukhi characters have direct similarities with Gujrati, Lande, Nagan, Sharda and Takn: they are either exactly the same or essentially alike.Internally, aara, haha, chacha, naana, dadda, nanna, naa, lalla letters of Gurmukhi had undergone some minor orthographical changes before AD 1610.
Further changes came in the forms of aaraa, haaha, and lalla in the first half of the nineteenth century. The manuscripts belonging to the eighteenth century have slightly different forms of these letters. But the modern as well as old forms of these letters are found in the orthography of the same writers in seventeenth and eighteenth centuries. Another reform carried out is the separation of lexical units of the sentence which previously formed one jumbled unit; lately punctuation marks borrowed from English have been incorporated besides the full stop (|) which existed traditionally.
The Gurmukhi script is semi-syllabic in the sense that 'a' is included in the consonant signs in some situations. This 'a' is not pronounced at the end of the syllable. Thus, kl (in punjabi) is kal, and RAM in punjabi is Ram, that is, k in (kal ) represents k+a, while l represents only l. Other vowels after consonants are shown by vowel symbols which also happen to be the first three letters of the Gurmukhl alphabet Of these, the first and the third are not used independently. They always have a diacritic attached to them. the second letter is used without diacritics also, and in that case it is equivalent to 'a' as in English 'about'. With diacritics a total of ten vowels are formed, viz., u, u, o, a, a, ai, au, i, l and e. Of these vocalic diacritics, 'i' occurs before a consonant (although pronounced after it), u and u are written below; a and l after a consonant; and e, ai, o and au over a consonant. Similarly, the nasalization sign is also used over a consonant though in fact it nasalizes the vowel. Of all the vowel-marks, called lagari in Punjabi, a is the oldest, though initially just a dot was used for it. The vowel-marks l and u are found in Asokan edicts and later inscriptions.
All Gurmukhi letters have uniform height and can be written between two parallel horizontal lines, with the only exception of e (the first letter of the alphabet) the top curve of which extends beyond the upper line. From left to right, too, they have almost uniform length, only of ( aira) and us (ghaggrha) may be slightly longer than the rest. However, the placing of vowel-symbols under and over the letters, a characteristic of all Indian scripts, creates some problems in printing and typing. No change is effected in the form of the letter when a vowel-symbol or diacritic is attached to it, the only exception being e to which an additional curve is added which represents two syllables. This is the only example of a single graphic form representing multiple sounds (and this form has a theological background); otherwise there is no Gurmukhl letter representing more than one phoneme, and there are no digraphs.
Gurmukhi has played a significant role in Sikh faith and tradition. It was originally employed for the Sikh scriptures. The script spread widely under Maharaja Ranjit Singh and after him under the Punjab Sikh chiefs, for administrative purposes. It played a great part in consolidating and standardizing the Punjabi language. For centuries it has been the main medium of literacy in the Punjab and its adjoining areas where earliest schools were attached to gurdwaras. Now it is used in all spheres of culture, arts, education and administration. It is the state script of the Punjab and as such its common and secular character has been firmly established.
The alphabet has also crossed the frontiers of its homeland. Sikhs have settled in all parts of the world and Gurmukhi has accompanied them everywhere. It has a brighter fixture, incleed, in and outside the land of its birth. Till recently, Persian script was largely used for Punjabi and there was initially a considerable amount of writing in this script, but it is becoming dated now. However, in the Pakistan Punjab Punjabi is still studied, at postgraduate level, in Persian script now called ShahMukhi.
:Encyclopaedia of Sikhism - Harbans Singh