The majority of hybrid electric vehicles (HEVs) available on the market are power-split hybrid vehicles with a single planetary gear (PG), including the popular Toyota Prius and Chevy Volt. Although both vehicles use a single PG, they have different configurations with different numbers of operating modes. The Prius has no clutch and has a single operating mode, whereas the Chevy Volt uses three clutches and has four modes. The goal of this paper is to present a thorough analysis of all possible configurations of power-split hybrid powertrain using a single PG. The analysis includes the following steps: 1) search for all possible ways to connect powertrain elements to the PG; 2) identify all potential locations for clutch installations around the PG and examine the feasibility of additional operating modes introduced by the clutch installation; and 3) optimize fuel economy for performance comparison. The proposed analysis shows that a single PG can produce 12 different configurations, each of which can have four feasible operating modes by adding three clutches to the PG. In case studies, we focus on the two configurations that are used in the Prius and Volt to find the impact of adding (or removing) clutches and modes on their fuel economy performance. Our results show that adding one clutch to the Prius transmission (which is named "Prius(+)") can significantly improve fuel economy in urban driving, whereas removing two clutches from the Volt transmission ("Volt(-)") will not significantly affect fuel economy in both urban and highway driving. This multimode configuration analysis can be used to systematically design future power-split HEVs.