First, languages that are thought to have arisen from a common proto-language must meet certain criteria in order to be grouped together; this is a process called subgrouping.
Since this grouping is based purely on linguistics, manuscripts and other historical documentation should be analyzed to accomplish this step.
This principle is again reflected when choosing the sound quality of phonemes, as the one which results in the fewest changes (with respect to the data) is preferred.
The Most Natural Development Principle states that some alterations in languages, diachronically speaking, are more common than others.
The Most Natural Development Principle describes the general directions in which languages appear to change and so one can search for those indicators.