In this paper, I discuss the emergence of articles, the and a/an, in the history of English. The indefinite article a/an appeared later than the definite article the in English. This phenomenon is also to be observed in other languages（Abraham 1997, Lyons 1999）. Furthermore, there are marked asymmetries between the definite and indefinite articles in terms of both semantics and distribution （Christophersen 1939, Lyons 1999, Crisma 2011, Dryer and Haspelmath 2013）. Here I try to elucidate the reasons for these asymmetries. The articles the and a/an are believed to have developed from the Old English demonstrative se/seo and the numeral an ‘one’（cf. Sommerer 2011）. This is an instantiation of grammaticalization（Hopper and Traugott 2003）. I assume that a syntactic determiner system, DP（Abney 1987）, was absent in Old English. Based on an examination of The York-Toronto-Helsinki Parsed Corpus of Old English Prose（YCOE）, and The Penn-Helsinki Parsed Corpus of Middle English, Phase II（PPCME2）, I claim that se/seo contributed to this grammaticalization（primary grammaticalization）, while an was grammaticalized as a result of this primary grammaticalization. In my hypothesis, grammaticalization means creating a functional space in a given structure. In this case, a space has been established before a noun in anominal structure. Therefore, although depending on the properties of nominals, the use of determiners has become obligatory in Present-day English（cf. Gelderen 1993, 2000）. Se/seo contributed to the creation of this space, while an was later grammticalized in the determiner space created by se/seo .Hence, the time difference in their appearance can be accounted for in this way.