The Lex Trebonia was a law passed in 448 BC to forbid the tribunes of the plebs from co-opting colleagues to fill vacant positions.
Appius Claudius Crassus, who had been consul-elect before the decemvirate, was the only member of the first college to participate in the second, and he ensured that his colleagues for the second year were like-minded and easily dominated by himself.
The tribunes of the plebs passed laws restoring the consular government, and making permanent both the right of appeal and the continuance of their own college.
The law was passed, and so effective was Trebonius at frustrating the patricians' designs during his year of office that he earned the surname Asper, meaning "prickly".
In this they failed, but two plebeians were still chosen as tribunes by co-optation, to the great annoyance of their colleague, Gnaeus Trebonius, whose name was attached to the flouted law.