Language & WorldHow it works
How linguists reconstruct a language nobody wrote down.
The comparative method: collect words of the same meaning from related languages, such as Latin 'pater', Greek 'patḗr', Sanskrit 'pitṛ́', Gothic 'fadar'. Look for regular sound correspondences: where Latin has p, Gothic has f, across many words. From such rules you derive the ancestral form, here '*ph₂tḗr'. Chance resemblance fails the test because it forms no rule.
Useful: One similar word proves nothing; only a pattern across dozens of words counts.
Source: en.wikipedia.org