The two lines, Agiad and Eurypontid are said to have originated from the twin brothers Eurysthenes and Procles. The Agiad line was considered to be the senior of the two, although this seniority was minimal.

The kings assumed two different roles, one in peace time and the other in time of war. In order to take a closer look at the origins and functions of the kings, we shall firstly take a look at Herodotus and his writings:

The Lacedaemonians declare, contradicting therein all of the poets, that it was king Aristodemus himself, son of Aristomachus, grandson of Cleodaeus, and great - grandson of Hyllus, who conducted them to the land which they now posses, and not the sons of Aristodemus. The wife of Aristodemus, whose name ( they say ) was Argeia, and whose daughter of Autesion, son of Tisamenus, grandson of Thersander, and great - grandson of Polynices, within a little while after their coming into the country, gave birth to twins. Aristodemus just lived to see his children, but died soon afterwards of a disease.

The Lacedaemonians of that day determined, according to custom, to take for their king the elder of the two children; but they were so alike, and so exactly of one size, that they could not popssibly tell which of the two to choose: so when they found themselves unable to make a choice, or haply even earlier, they went to the mother and asked her to tell them which was the elder, whereupon she declared that "she herself did not know the children apart;" although in good truth she knew them very well, and only feigned ignorance in order that , if it were possible, both of them might be made kings of Sparta.

The Lacedaemonians were now in a great strait; so they sent to the Delphi and inquired of the oracle how they should deal with the matter. The Pythoness made answer, "Let both be taken to be kings; but let the elder have the greater honour." So the Lacedaemonians were in as great as strait as before, and could not conceive how they were to discover which was the first - born, till at a length a certain Messenian, by name Panites, suggested to them to watch and see which of the two the mother washed and fed first; if they found she always gave one the preference, that fact would tell them all they wanted to know; if, on the contrary, she herself varied, and sometimes took the one first, sometimes the other, it would be plain that she knew as little as they; in which case they must try some other plan.

The Lacedaemonians did according to the advice of the Messenian, and without, letting her know why, kept a watch upon the mother; by which means they discovered that, whenever she either washed or fed the children, she always the same child the preference. So they took the boy whom the mother honoured the most, and regarding him the first - born, brought him up in the palace; and the name which they gave to the elder boy was Eurysthenes, while his brother they called Procles. When the brothers grew up, there was always, so long as they lived, enemity between them; and the houses sprung from their loins have continued the feud to this day.