Jaiminīya (Talavakāra) Brāhmaṇa 7.25

Upon them [is sung] the Pārtha[-sāman], having a twofold repetition, the form of the second day, having a refrain, in bṛhatī form;
therefore it is performed on the day in bṛhatī form.
Pṛthu Vainya desired: "May I be consecrated to the kingship over all cattle!"
He saw this sāman.
He praised with it.
Then indeed he was consecrated to the kingship over all cattle.
This is a sāman of splendor.
He is consecrated to the kingship over all cattle, who knows thus.
And it has a refrain-prelude (stobhaprastāva) and is endowed with tone (svara), for the division of tones.
For that is the svāra-tone of this day, which has a refrain-prelude.
Rising upward as it were is this day, rising upward as it were is this sāman.
He mounts the day itself by this, while chanting praise.
And because Pṛthu Vainya saw [it], therefore it is named Pārtha.

They say, however, that the Vāsiṣṭha is more to be performed.

This is royal power—that it is the Pārtha; this is royal power—that they are the two Soma-sāmans; this day is royal power; royal power in that

it belongs to Indra; royal power in that it is in triṣṭubh meter; royal power in that it is in bṛhatī form; royal power in that

the stoma is fifteenfold.

Royal power would thus be uniform (ekadhā).

Now, because the Yauktāśva is in the beginning, and the Vāsiṣṭha later, the two seer-sāmans are for the counterbalancing of the brahman,

[and]

so that the brahman may be joined with royal power.