Historical usage
In Old English, that did not exist, and was only represented by þe. [10] It originated in the north of England sometime before the 1200s and spread around the country in the thirteenth century; it then rapidly became the dominant demonstrative pronoun. [11] Before the writings of Ælfric of Eynsham, þæt was normally regularized as þe in writing, but by the time Ælfric lived, þæt was common. [12] As a pronoun, þæt was widely used in Old English, though it was later replaced by wh- words. [10] Where þe had only stood in for subjects of a clause, þæt instead took on the role of both a subject and an object, [13] and when þe and þæt were both used, þæt was always relative in orientation. [14] The symbol ꝥ ( 
) was used as an abbreviation, before it was phased out by the Romantic þt. [15] Similarly, yͭ was a ligature to represent that, [16] as seen in the gravestone of William Shakespeare: "Bleste be yͤ man yͭ spares thes stones". [17] In Middle English, þe was entirely replaced by þat (among other representations), before again being replaced by the modern that. [10] Among all relative markers in the English language, including who , which, whose, and what, that—through its ancient form of þæt—appears to be the oldest. [11]
In Old English translations of Latin (but only sparsely in original Old English texts), the phrase þæt an is frequently used—typically meaning "only"—but its origins and characteristics are not well-understood. [18] Frequently, the construction of þæt an was in the original Latin, which referred then to a following clause. [19] The use of þæt an was for cases in which there was exclusivity (to distinguish between general and specific objects), but translators also used it in situations where exclusivity was already given through other syntactical elements of the sentence. [20] In these texts, þæt seems to be used pleonastically (redundantly), and it began to be used as an independent adverb. [21] In the context of weather events, þæt was never used, such as in the example sentence þæt rigneð (translated as "that rains"). [22]
Similarly, for several centuries in Old English and early Middle English texts, the phrase onmang þæt (translated as "among that") persisted. [23] In the hundreds of years of its existence, it was used infrequently, though the usage was stable. [24] Even in Old English, usage of hwile ("while") was much more commonplace, with its frequency some six times as large as onmang þæt in a surveyed corpus. [25] Onmang þæt experienced grammaticalisation (turning a word into a grammatical marker), [25] and as a result of its low usage, possibly underwent a period of specialization, where it competed with other grammaticalised phrases. [26]
After verbs such as said, and more generally in introducing a dependent clause, contemporary English grammar allows the speaker to either include that or to omit it. [27] This construction—as in "I suspect (that) he is right"—is called the zero form when that is not used. [27] While there has been some analysis of the relative frequency of Old and Middle English usage of the zero form, these studies are of limited value, since they rely on unique text corpora, failing to give a general view of its usage. [28] In the late period of Middle English, the linguist Norihiko Otsu determined, the zero form was generally as popular as the form in which that is included. [29] The zero form was common in documents closely relating to speech, such as sermons, suggesting spoken English often omitted that in these contexts. [30]
