Recently I asked a [question][1] on the naming of polyhydroxy phenols. One of them was [**pyro**gallol][2] (1,2,3-Trihydroxybenzene) which is  synthesized from [gallic acid][3]  obtained from oak galls. (BEWARE- not a compound of gallium). 
[![enter image description here][4]][4]


But why "pyro"? Pyro is usen in naming inorganic acids like **pyro**sulfuric acid ($\ce{H2S2O7}$) or **pyro**phosphoric acid ($\ce{H4P2O7}$). It is also observed that the carboxylic acid derivative is gallic acid not pyrogallic acid and the hydroxy derivative is pyrogallol not gallol. Why is it so?


  [1]: http://chemistry.stackexchange.com/questions/43890/why-is-this-compound-named-resorcinol
  [2]: https://en.wikipedia.org/wiki/Pyrogallol
  [3]: https://en.wikipedia.org/wiki/Gallic_acid
  [4]: https://i.sstatic.net/NFtj1.png