This thiruneeru* is an important sign of the people who follow the Saivite procedures. It is also followed as a one of the sign of Lord Siva. In the thiruneeru, that we apply by using the three fingers, the three straight lines represents the thought, word and action. It also specifies about the point that everybody's life ends up as ashes only.
The Saivites consider the committed devotee who keeps the thiruneeru as Lord Siva only. A washerman whose body is covered of crude carbonate of soda was considered as Lord Siva and bowed and prayed by Chera King, who is one amongst the Naayanmaar called as Cheramaan Perumaal Naayanaar.
Meiporul Naayanaar and Yaenaadhi Naayanaar are the two who sacrificed their life for thiruneeru which is considered as physical form of Veda.
To cure the sunstroke of Koon Paandi, Thirugnana Sammandhar has sung a song explaining the greatness of the thiruneeru.
In this way,
"கங்காளன் பூசும் கவசத்திரு நீற்றை
kangaalan poosum kavasaththiru neetrai
மங்காமற்பூசி மகிழ்வரே யாமாகில்
mangaamarpoosi magizhvarae yaamaakil
தங்க வினைகளும் சாருஞ் சிவகதி
thanga vinaikalum saarunj sivakathi
சிங்காரமான திருவடி சேர்வரே
singaaramaana thiruvadi saervarae",
Thirumoolar has also sung the above song explaining the greatness of thiruneeru.
"ஆசை கெடுப்பது நீறு அந்தம தாவது நீறு
aasai keduppathu neeru anthama thaavathu neeru"
*Thiruneeru - the sacred ashes (used for Saivite mark)
Original - www.siththarkal.com