In the interregnum after the death of Romulus the Roman Senate, comprised at that time of 100 men, arranged itself into ten decuries, and each decuria governed Rome for five days.
In a rotating manner, each man within a decuria reigned for 12 hours, six by day and six by night, as interrex.
[1] The decuriae continued to rotate the government amongst themselves for a year until the election and accession of Numa Pompilius.
[2] Decuria was also a Roman unit of measurement applied to civitas of native peoples.
From *dek- 'ten' plus *-ur-yo-, a Common Italic formation, judging from the related Umbrian dequrier, tekuries 'decurial'; and compare Oscan dekkviarim.