本文分析「許」字在北魏．賈思勰《齊民要術》一書中的用法，得出共有「方位詞後綴」、「概數詞」及「如＋NP＋許」結構等幾種不同的用例，並分別論證其來源與發展，指出方位詞後綴「許」來自於較早的處所詞用法，更具體地說，則是來源於上古漢語的處所詞「所」。到了後來，則演變為方位詞的後綴，形成「內許」、「外許」等詞語。概數詞「許」基本上也是來自於上古漢語表不定數的「所」，並且進一步取代「所」字此一語法功能。至於「如＋NP＋許」結構中的「許」，則是同時受到表處所「許」及表概數「許」的影響，使其出現在「如＋NP＋許」句式之中，並具有表達平比的語法功能。

This article analyzes the usage of ”xu” in ”Qi Min Yao Shu” written by Jia Sixie in Northern Wei Dynasty. We found that ”xu” had ways of usage like ”the suffix of the directional words”, ”Approximate word” and the sentence pattern of ”ru + NP + xu” etc. We also discuss its origin and the development separately and point out that the function of ”the suffix of the directional words” came from the earlier usage of the locative word. More specifically, the source of the suffix ”xu” was originally from an ancient Chinese word ”suo”. To the later, the word evolved into the directional suffix which formed the words of ”nei xu” and ”wai xu”. Approximate word ”xu” also comes from an ancient Chinese word ”suo” which was used to mean approximate number basically, and further to replace the grammatical functions of the approximate word ”suo”. As for the word ”xu” with the function of comparison appears in the sentence pattern of ”ru + NP + xu”, was simultaneously affected by the locative ”xu” and approximate ”xu”.