Morphological agreement with coordinate phrases involves a computation that takes as its input a set of features from each conjunct and outputs exactly one resolved set of features (number, person, grammatical gender/noun class, commonly labelled phi-features). Such resolution is typically taken to be grammar-internal because it relies on other grammar-internal ingredients (phi-features, agreement, coordination), and at least in some instances seems to follow systematic rules that may be captured by familiar grammatical operations. Exceptions to these apparent rules, if not ignored altogether, have received disparate analyses depending on the language, framework, and the particular features involved. In this thesis, I argue that it is such exceptions that are illuminating, and that the appearance of rigid rules is misleading. Treating variation in agreement with coordinate phrases as exceptional with respect to the otherwise deterministic output rules either delays the task of explaining the surface data, or risks weakening the language competence theory by adding the baroque stipulations that a purely grammar-internal treatment would require. Phi-agreement with coordinate phrases is subject to inter- and intra-speaker variability and ineffability; such variation is widespread in the world’s languages, even the ones with limited phi-agreement morphology like English. I therefore reject the grammar-internal approach to agreement with coordinate phrases and argue instead that the agreement morphology we observe on the surface is due to grammar-external mechanisms being recruited to determine the resulting agreement morphology. Under this approach, systematicity in agreement with coordinations is only apparent and can be manipulated. The reason a grammar-internal mechanism is unavailable is because it would have to take place on the agreeing head (e.g., Infl0 or v0), and what we know about agreement between a syntactic head and multiple arguments (e.g., from omnivorous agreement and the Person Case Constraint) renders it ill-suited for the task of coordination resolution.