雖然語言中有絕大部分來自於慣用語句，但慣用語在語言學上是個棘手的議題--我們無法僅由字面上得知其真實的意涵。近來由於結構語法學的興起，我們漸漸意識到，慣用語在語法、語意及語用上的非預測性應求諸於結構本身。基於這樣的理念，本篇文章以中文四字詞 “X-來-X/Y-去” (XLXQ or XLYQ), “不-X-不-Y” (BXBY)，及 “不-X-而-Y” (BXEY) 為研究題材，提出四字詞的存在代表了語言中某種心理的呈現，亦即認知結構的理念。這三種四字詞反映出不同程度的語法化現象。經由研究這三種四字詞的語言特性及結構意涵，得知其意義不僅來自於其組成的詞素。句法，語意甚至語用層次的知識都須納入語言分析中，以求得字面之外，得自於“非字詞成分＂的語意來源。

While idioms and idiomatic patterns constitute a large part of our linguistic knowledge, they nevertheless present special difficulties because their meanings are not predictable merely from the combination of their constituents. Recently, as studies of construction grammar (CG hereafter) has suggested, the syntactic, semantic and pragmatic unpredictability of the so-called constructional idioms should be resorted to Construction itself. Based on the CG assumption, this paper aims to suggest that the existence of Constructions such as the Chinese X-lai (come)-X/Y-qu (go) (XLXQ or XLYQ), bu (not)-X-bu (not)-Y (BXBY), and bu (not)-X-er (yet)-Y (BXEY) stands for a certain format of mental representation, i.e., the conceptual structure. The difference in terms of their degree of productivity hints at their different stage in the grammaticalization process. The comparison of the linguistic features and constructional meanings of these three combinations will show that one does not interpret the idiomatic expression solely on the lexicon. Levels beyond syntax, semantics and even pragmatics should be incorporated in linguistic analysis so that a speaker may grasp meaning of “non-lexically filled elements” that are not in the lexicon. For this reason, a constructional idiom is taken as “a match of syntactic and conceptual structure” which cannot be derived compositionally.