Code-switching (CS) has been defined as ‘the act of alternation of two languages within a single discourse, sentence or constituent’ Poplack (1980:583). Technically, code-switching refers to alternation between sentences while intra-sentential alternation is referred to as code-mixing. Following Poplack (1980), we use the term code-switching to refer to the intra-sentential alternation which is more amenable to grammatical description. Most scholars on codeswitching will agree that it has a grammar but differ on the nature of this grammar. Some are of the opinion that codeswitching involves two monolingual grammars with constraints that determine points where switches are possible. While some see code-switching as a monolingual grammar where the host or base language provides the mould into which lexical items from both languages could be inserted. The present researcher belongs to the last school of thought. Adopting the Functional Head Selection Constraint of the Matrix Language Frame (MLF) Model proposed by Myers- Scotton (1993, 1995) and with samples of code-switched expression collected from Igbo English bilinguals, the study examines the pattern of code-switching using Igbo and English as the focal point. Igbo is the Matrix language while English is the Embedded language. Our study reveals that the functional heads such as T, Agr, Neg, Det, would normally come from Igbo and provide the skeletal mould. The choice of whether the lexical items will come from Igbo or English or both is determined by the selectional properties of the Igbo functional heads. Perhaps this is the case for all other language pairs where one language is the Matrix Language (ML) and the other Embedded Language (EL).UJAH: Unizik Journal of Arts and Humanities vol 14 (2) 2013