This thesis develops a theory of Chain Uniformity based on a strict A/A-Bar distinction, replying to alleged shortcomings of an A-A-bar typology of syntactic positions. Arguing all positions within Functional Categories (FCs) are inherently undetermined w.r.t. A/A-bar, chain Uniformity allows a chain contextual determination of their A/A-bar status, eliminating the exponential complexity related to increasing number of FCs from a language acquisition viewpoint, yet allowing a cross-linguistic flexibility previous typologies lacked. Chain Uniformity implies a reanalysis of (non-unifrom) Operator-variable chains as two uniform chains connnected through Clausal predication (as in Null Operator constructions (NOCs)) at the level of AgrPs, capturing intricate properties of past-participle agreement in French (chapter 3). Other chapters extend the analysis to specific constructions, e.g. scrambling (chapter 5), Weak and Weakest crossover effects (chapter 4) and NOCs (chapter 6).