Andrew Carnie presents Syntax: A Generative Introduction. 4th Edition (2021). Wiley Blackwell.
Video 19.1: External Merge
Chapter 19 looks at the Minimalist theory of Merge, which tries to derive X-bar theoretic notions from otherwise motivated things we need. X-bar is shown to derive from the mathematical notion of set formation and the differences between X heads, X's, and XPs, follows from the domination relationships. The operation of External merge is explored.
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