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The clearest cases are those where the parent language is known to exist. For example, on the basis of various words for “father” in the Romance languages, it is possible to see how they all derived from the Latin word “pater”. If Latin no longer existed, it would be possible to reconstruct a great deal of its form, by comparing large numbers of words in this way.
Virtually Any Structural Feature, Morphology or Word-Structure, Morphemes, Languages and Establishes, Comparative Typology, Comparative Grammar, Confrontational Grammar, Descriptive-Comparative Linguistics