The Bhikkhus Rules
A Guide for Laypeople
Names And Forms Of Address
During his ordination, the bhikkhu candidate is asked formally for his name. His Preceptor (usually) will have given him a Paali name and this is what he will use. However, later, on less formal occasions, he may be addressed differently. This variety of terms of address can be quite confusing for outsiders. For example, in Thailand, the monk will more often use his given name (from before his ordination) with an honorific preceding it appropriate to his monks seniority and rank. The Paali name, and title if any, would be added on more formal occasions. I understand that in Sri Lanka, and sometimes in Burma, it is the bhikkhus place of origin or residence that may be prefixed to his Paali name.
Some monks may use the description Bhikkhu before their Paali name (Bhikkhu X) while others will use it as a suffix (X Bhikkhu). If they are more than ten years in the robe they may use Thera (Elder) and if very senior Mahaathera. (See also Becoming a Bhikkhu.)
There are many other titles and ranks for senior bhikkhus. The king (in Thailand) or government often confer these in recognition of service or administrative ability. When administration of all the bhikkhus of the country is subsumed under central government departments, it may then be divided up into regions and districts under the supervision of the local senior respectable monks. However, underlying all of this is the Vinaya Rule that still guides the traditional ways of the bhikkhu life, without class or privilege, and it remains the foundation for continued Dhamma practice as it has done for the last twenty five centuries.
Probably the most universally acceptable form of address for any bhikkhu is "Bhante" or "Venerable Sir."
Footnotes and references:
In Thailand, the common honorifics in (roughly) ascending order of age and seniority are: Tan, Phra, Luang Pee, Kruba, Ajahn, Tan Ajahn, Luang Por, Luang Poo. The Thai titles of ascending ecclesiastical rank are: Phra Khru, Chow Khun, Somdet, and Somdet Phra Sangha Raht (or the Supreme Patriarch). Also note that the English transliteration of these Thai titles also varies, for example, Acharn, Ajaan, Ajahn.