From the Brihat Parashara Hora Shastra, ch. 36, verse 11-13. Translated by Girish Chand Sharma:
A raja yoga is a planetary combination that gives increase in success and status. Here sage Parashara shows the various ways that a raja yoga can be formed:
(verses 11-12) If there is an exchange between an angular lord and a trinal lord, or if they are in conjunction in a house, or if either of them is there in the other's house, or if they have full aspect of each other, they are indicators of Raja Yoga (they cause auspicious combinations). The native born in this combination will become a king or an eminent person.
These rules are of the greatest significance in giving elevation to life. The basis of Predictive Astrology is indeed the mutual relationship of the lords of the houses. The above mentioned rules may be easily explained as follows:
1) The angular lord occupies a trine and the trinal lord occupies an angle. That is, there should be an exchange between an angle and a trine.
2) The lords of an angle and a trine are situated together in an angle or in a trine.
3) If a trinal lord is situated in an angle, he is always in an auspicious situation although he may not be in his own sign.
4) If an angular lord occupies a trine he will always be in an auspicious situation. His situation in his own sign is not essential for him to be auspicious.
5) The angular lord being situated in any house aspects the trinal lord situated in a benefic house and the trinal lord aspects the angular lord in the same position or they have aspects between them.
The greater this relationship of the planets there is in a horoscope the more famous, the wealthier and the more powerful will the native be.
(verse 13) If one and the same planet becomes the lord of a trine and also the lord of an angle and occupies an angle or a trine, it will prove to be a raja yoga karaka.
This situation may be created by Saturn in Libra and Taurus ascendants because then he will be respectively the lord of the 4th and 5th houses and that of the 9th and 10th houses. And if Saturn in these ascendants occupies an angle or a trine he will form a specially auspicious yoga or combination.