In the Buddha's time, Buddhism was usually called, "This Dhamma-Vinaya" (the Doctrine and the Discipline). This means that the Dhamma-Vinaya is another name, actually the original name, for Buddhism. It also indicates that the Dhamma and the Vinaya are the two limbs that form Buddhism. This concept is in direct connection with the two kinds of Sangha.
The Conventional Sangha of monks depends for its existence and stability on the Vinaya. It is the Vinaya that gives life to the Bhikkhu-Sangha. A person is ordained a Bhikkhu or is admitted to the Sangha of monks in accordance with the rules of the Vinaya. His Bhikkhuship also ceases if he makes an incurable transgression against the Vinaya. The rules of the Vinaya govern all activities of the community of monks and all aspects of a Bhikkhu's life. Monks or Bhikkhus are graded according to concrete disciplinary rules. A Bhikkhu is classed as a Navaka, or a newly ordained one, if he has been admitted to the Order for not more than five years, and he is required to live in "Nissaya," or dependence, on an Ācariya or teacher, that is, he is a dependent. As soon as his years of standing in the monkhood exceed five, he becomes freed of the Dependence and is classed as a Nissaya-Muttaka, or an independent monk. When he completes ten years of standing in the monkhood, he becomes a Thera or an Elder. Now, if he is qualified, he can act as an Upajjhāya or a preceptor. Rights and privileges are vested on the Bhikkhus on equal terms according to the rules of the Vinaya.
The Noble Sangha of disciples, on the other hand, is based on the Dhamma. While the Vinaya governs the external life of a monk, his bodily and verbal activities and his social relations with others, his inner and spiritual side is guided by the Dhamma. Not only the monks, but all people are expected to follow the guidance of the Dhamma. In contrast to the formal admittance to membership in the conventional Sangha of monks, membership in the Noble Sangha of disciples is a matter of self-development and inner attainment. As soon as a person, whether a monk or a layman, realizes the Four Noble Truths and gains a first vision of Nibbāna, he automatically becomes a Sotāpanna and, simultaneously, a member of the Noble Sangha of disciples. His further progress on the path up to the final goal is graded solely according to the degree of his self-development and inner attainment, without the intervention of any external factors, whether age, sex, authority or even time or space. Thus, a novice twelve years of age may be an Arahant while an aged monk seventy years old may be only a worldling, not attaining even Sotāpannaship, and a wise layman may achieve Arahatship in a period of an hour while many monks may strive in vain throughout their lives to secure the same. As the Buddha says in the Dhammapada: He who, though dressed in fine apparel, exercises tranquillity, is calm, controlled, certain and chaste and has ceased to injure all other beings, he indeed is a Brāhman, a Samana, a Bhikkhu. (Dh.142)
Though the treading of the path of self-development and inner attainment is a personal task, the treader is not all alone or helpless. Besides the Great Teacher, the Buddha, who shows him the Way and equips him with the tools, the conventional Sangha of monks, as regulated by the Vinaya, provides him with Kalyāṇamitta, or good spiritual friends, who will counsel and encourage him along the Way, with a way of life and living conditions that are advantageous to his endeavor. In particular, those members of the Noble Sangha who are far advanced on the Path or have reached the summit, will find the Bhikkhu-Sangha the best community for them to live in, and it is these people who will best preserve the conventional Sangha of monks and will act as Kalyāṇamitta, or good spiritual friends, to those who are treading the Path after them.
In short, the two kinds of the Sangha are reciprocally helpful and complementary in the realization of Buddhist ideals. Without the will and effort to join or to maintain the Noble Sangha of disciples, the conventional Sangha of monks is meaningless or, at least, strays away from the ideal set up by its Founder, the Buddha. Without a concrete organization like the Bhikkhu-Sangha as the tool, the task of establishing and maintaining the Ariya-Sangha of disciples would be very difficult, if not an impossibility.