Tlor Thakal was a type of clothing that was used in Vulcan society.
This ceremonial attire varied between individuals but consisted of hat, shirt, vest, pants or skirt along with boots and a hip long robe. Every Vulcan wore a Tlor Thakal when participating in important gatherings or rituals. Due to the formality of meetings of the Federation Council and Assembly, it was a requirement for a Vulcan diplomat to wear their Tlor Thakal.
The history of this form of garment dated back to Vulcan's violent past. During those times, the position of messenger was despised due to the fact that those who took such a position travelled back and forth across the battlefield where they delivered a warlord's threat or an insult to an enemy. Those that were chosen this role were typically mentally handicapped and those who did not want to fight. In the eyes of warriors of early Vulcan, to not fight was to blaspheme against the gods and thus pacifistic creatures were lower then any animal in a dung heap. This animosity towards the messengers meant their deaths both on the battlefield and off it.
At some point, a warlord decided to take measures to safeguard his messengers. Partly to ensure they lived and partly to satisfy his own sick form of humor, the warlord made fools design a costume for his messengers which was called "the Uniform of Deep Thinkers". Whilst this meant that messengers were no longer confused with the enemy, it meant that they suffered from the weight of their clothing and the oppressive heat of their home world.
When the philosopher Surak first ventured onto the plains in order to preach his message of peace and logic, he did so in Tlor Thakal. Whilst he negotiated peace, his physically strong form meant that he wore the clothing with such diginity that none confused his clothing to be that of a fool. Surak himself wore the Tlor Thakal as a matter of respect to the messengers, calling them real Vulcans. He also extolled the Tlor Thakal as a way of ignoring the body and focusing on the mind. Thus, in future generations, Vulcans that wear this attire were reminding themselves of Surak's life as well as his devotion to logic. (FASA RPG module: Star Trek IV Sourcebook Update)