SPQR

SPQR is the motto of the old Roman Empire. It stands for  S enatus P opulus q ue R omanus. In English, it translates to: "The Senate and the People of Rome".

The Lost Hero
The initials SPQR is seen on Jason's arm with an eagle (Jupiter's symbol) and 12 bar lines.

The Son of Neptune
All centurions are seen with the SPQR tattoo along with their godly parent's symbol and at least one bar line. Reyna's tattoo consists of the initials SPQR, a crossed sword and torch, the symbol of her mother, Bellona and 4 bar lines, representing her four years of service. Hazel's consist of the same initials, a hieroglyph-like symbol of Pluto, and one bar line. At the council, before their quest, Frank receives his tattoo with Mars' symbol, crossed spears, and one line. Near the end of the book, along with being voted as praetor, Percy also receives his tattoo, this time with a trident, Neptune's symbol, and a line.

Usage
When a camper at Camp Jupiter is no longer on probatio and is accepted into the legion, they are branded with the symbol of their godly parent or ancestor (if they are a legacy, like Octavian with lines underneath, showing their years of service to the Legion.