This paper examines the legal ramifications of using tariff flexibility arising from GATT unbound tariff lines or tariff overhangs under both WTO and preferential trade agreement (PTA) law when flexibility is exercised preferentially for PTA partners. Under WTO law, a WTO member that is a party to a PTA under GATT Article XXIV is required to use tariff flexibility on a non-discriminatory basis. However, PTA obligations including tariff elimination commitments and a standstill clause prohibit the WTO member from raising applied tariffs on imports from the PTA parties, thereby preventing the use of tariff flexibility on a non-discriminatory manner. In contrast, a WTO member entering into a PTA under the Enabling Clause may use tariff flexibility discriminatorily without violating WTO law. A WTO member has greater legal latitude in utilizing tariff flexibility if it forms a PTA under the Enabling Clause rather than under GATT Article XXIV. The discriminatory exercise of tariff flexibility by some WTO members in conformity with PTA obligations, but in violation of WTO law, is further evidence of the fragmentation of the world trading system, where WTO law is facing its limits.