1)The protolanguage is the original form of a language family that has ceased to exist.
2)Historical linguists try to reconstruct the proto form in the common ancestral language by means of identifying and comparing simillar linguistic forms with similar meanings across related language, which is known as a method of comparative reconstruction.
3)Because the relatonship between the form and meaning of a word is arbitrary, the existence of systematic resemblance in the phonological and morphological forms and semantic notions of two or more languages are likely to have descended from a common ancestor.
4)So work on the systematic form-meaning resemblance in cognates, words that have descended from a common source lies at the core of comparative reconstruction. For example, English "mother, " "father" and "friend" are believed to be cognates of German "Mutter," "Vater" and "Freund." Historical linguists then hypothesize that these cognate sets in Modern English and Modern German are descendants of a common ancestor.