Our teacher, the fourth guide of this fortunate aeon, the incomparable lord of sages, Shakyamuni, gave infinite teachings as means to enter the Dharma of the causal and resultant vehicles, in accordance with the particular temperaments, spiritual faculties and attitudes of disciples. Nevertheless, they may all be included within the three vehicles, which, in turn, may be further subdivided into nine successive stages.

As it is said in The General Sutra:

The ultimate definitive vehicle
Certainly appears as three in number:
The vehicles of leading from the origin, Vedic asceticism,
And powerful transformative methods.

And The Immaculate Confession Tantra says:

The samayas of the nine successive yanas—
Three yanas related to the three pitakas of characteristics,
The outer three of kriya yoga and so on, related to tantras of asceticism,
And the inner three yogas related to tantras of skilful methods.

Thus the classification of ‘nine successive yanas,’ which is found in the Nyingma Early Translation tradition, is made up of:

• three yanas related to the outer vehicle of leading from the origin [of suffering] and the three pitakas of characteristics,
• three yanas related to the inner vehicle of Vedic asceticism2 and the three outer classes of tantra, and
• three yanas related to the secret vehicle of powerful transformative methods and the three inner classes of tantra.