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In historical linguistics, an asterisk immediately before a word indicates that the word is not directly attested, but has been reconstructed on the basis of other linguistic material (see also comparative method). The asterisk shows that this form of word is hypothetical and historically it could be more or less different.
Asterisks are therefore used mostly in the reconstruction unwritten or prehistorical languages, deduced by comparison between other languages.
For example the Old English word endleofan suggests a Proto-Germanic form was *ainlif. The Proto-Germanic people did not have a writing, therefore ainlif is unattested, and this is indicated with an asterisk.
Tolkien used asterisks when writing about the history of his Elvish languages, to show that for some reason, these forms are "unattested", and belong in a stage of a language before the Tengwar or the Cirth. As such, an asterix prefixed to a word indicates that the form is "hypothetical". As noted by Christopher Tolkien, such a marking means that the form is "deduced to have existed from later, recorded forms". Asterisks are used most usually in Primitive Quendian.
The purpose of Tolkien using asterisks for his personal writings is not known, and it is possible that he used them as a personal in-joke and to persuade himself that he worked as a historian; or perhaps in order to keep a personal note that these forms were invented by being based on the "later" forms of the words, and could change in some future revision.
In fandom, even the asterisked forms of Tolkien are considered "valid" and as canonical as the unasterisked ones.
Students of Tolkien's languages also use asterisks to show that some forms are hypothetical logically deduced from Tolkien's derivation of the languages, although Tolkien himself didn't write that form in any of his published texts.
For example the word *corma is a hypothetical word believed to mean "ring". It is seen only as part of the word cormacolindo meaning "Ring-bearer". The form of the longer word makes it obvious that a word "corma" probably exists in Quenya. But since the word as it is, is never written clearly by Tolkien, it is considered as hypothetical and marked with an asterisk.
Asterisks are also used to show ancient forms of attested words. The Sindarin word hobas from Root KHOP points to an Old Sindarin intermediate *khopasse. Tolkien never wrote this word, but the rules of Sindarin etymology makes this form the only possibility; however proper referencing requires an asterisk to make clear that it is not a word given by Tolkien.
Students also sometimes employ a double asterisk ** to indicate a mistaken hypothetical form to clarify an example. A mistaken form would be Primitive Quendian **aladâ 'tree' instead of the correct *galadâ. They both would bring Quenya alda but the former would bring Sindarin *aladh instead of the proper galadh. This example, although wrong, could clarify the behavior of g in the two languages.
Tolkien Gateway has entries only for attested canonical words. Asterisked forms will be given as examples, but will not have links to entries of their own.