Tamil script based on Tamil-Brahmi
History
The Tamil script, like the other Brahmic scripts, is thought to have evolved from the original Brahmi script. The earliest inscriptions which are accepted examples of Tamil writing date to the Ashokan period.  The script used by such inscriptions is commonly known as the Tamil-Brahmi or "Tamili script" and differs in many ways from standard Ashokan Brahmi. For example, early Tamil-Brahmi, unlike Ashokan Brahmi, had a system to distinguish between pure consonants (m, in this example) and consonants with an inherent vowel (ma, in this example). In addition, according to Iravatham Mahadevan, early Tamil Brahmi used slightly different vowel markers, had extra characters to represent letters not found in Sanskrit and omitted letters for sounds not present in Tamil such as voiced consonants and aspirates. Inscriptions from the 2nd century use a later form of Tamil-Brahmi, which is substantially similar to the writing system described in the Tolkāppiyam, an ancient Tamil grammar. Most notably, they used the puḷḷi to suppress the inherent vowel. The Tamil letters thereafter evolved towards a more rounded form and by the 5th or 6th century, they had reached a form called the early vaṭṭeḻuttu.The modern Tamil script does not, however, descend from that script. In the 4th century, the Pallava dynasty created a new script for Tamil and the Grantha alphabet evolved from it, adding the Vaṭṭeḻuttu alphabet for sounds not found to write Sanskrit. Parallel to Pallava script a new script (Chola-Pallava script, which evolved to modern Tamil script) again emerged in Chola territory resembling the same glyph development like Pallava script, but it did not evolve from that. By the 8th century, the new scripts supplanted Vaṭṭeḻuttu in the Chola resp. Pallava kingdoms which lay in the north portion of the Tamil-speaking region. However, Vaṭṭeḻuttu continued to be used in the southern portion of the Tamil-speaking region, in the Chera and Pandyan kingdoms until the 11th century, when the Pandyan kingdom was conquered by the Cholas.With the fall of Pallava kingdom, the Chola dynasty pushed the Chola-Pallava script as the de facto script. Over the next few centuries, the Chola-Pallava script evolved into the modern Tamil script. The Grantha and its parent script influenced the Tamil script notably. The use of palm leaves as the primary medium for writing led to changes in the script. The scribe had to be careful not to pierce the leaves with the stylus while writing because a leaf with a hole was more likely to tear and decay faster. As a result, the use of the puḷḷi to distinguish pure consonants became rare, with pure consonants usually being written as if the inherent vowel were present. Similarly, the vowel marker for the kuṟṟiyal ukaram, a half-rounded u which occurs at the end of some words and in the medial position in certain compound words, also fell out of use and was replaced by the marker for the simple u. The puḷḷi did not fully reappear until the introduction of printing, but the marker kuṟṟiyal ukaram never came back into use although the sound itself still exists and plays an important role in Tamil prosody.
The forms of some of the letters were simplified in the 19th century to make the script easier to typeset. In the 20th century, the script was simplified even further in a series of reforms, which regularised the vowel markers used with consonants by eliminating special markers and most irregular forms.