[a] The main role of these words is to mark a clause as interrogative.
[1]: 686 Semantically speaking, when used in a main clause, the interrogative words do not refer but rather question.
In contrast, whether and if, like other subordinators, have no semantic value, and simply mark the clause as interrogative.
Ultimately, the English interrogative words (those beginning with wh in addition to the word how), derive from the Proto-Indo-European root kwo- or kwi,[4] the former of which was reflected in Proto-Germanic as χwa- or khwa-, due to Grimm's law.
[5] These underwent further sound changes and spelling changes, notably wh-cluster reductions, resulting in the initial sound being either /w/ (in most dialects) or /h/ (how, who) and the initial spelling being either wh or h (how).
[5] In how (Old English hū, from Proto-Germanic χwō), the w merged into the lave of the word, as it did in Old Frisian hū, hō (Dutch hoe "how"), but it can still be seen in Old Saxon hwō, Old High German hwuo (German wie "how").
In English, the gradual change of voiceless stops into voiceless fricatives (phase 1 of Grimm's law) during the development of Germanic languages is responsible for "wh-" of interrogatives.
Although some varieties of American English and various Scottish dialects still preserve the original sound (i.e. [ʍ] rather than [w]), most have only the [w].
[5] The words who, whom, whose, what and why, can all be considered to come from a single Old English word hwā, reflecting its masculine and feminine nominative (hwā), dative (hwām), genitive (hwæs), neuter nominative and accusative (hwæt), and instrumental (masculine and neuter singular) (hwȳ, later hwī) respectively.
[6] Other interrogative words, such as which, how, where, whence, or whither,[7] derive either from compounds (which coming from a compound of hwā [what, who] and līc [like]),[8] or other words from the same root (how deriving from hū).