All human languages have some things in common, but certain languages--because of their shared history--have a great deal in common. Those commonalities can help a learner proficient in one of a pair of related languages learn the second language more quickly. This book will serve as a catalyst for adult English speakers who have learned either Spanish or French as an L2 and wish to learn the other language as an L3. The manual takes advantage of the fact that learners with intermediate proficiency in an L2 are used to thinking consciously about language, know themselves as language learners, and can capitalize on what they know about one language to understand the other. The manual contains seven chapters that present concise and linguistically accurate descriptions of the paired languages, provide targeted exercises with answer keys, and give many examples of each principle. Audio files help with pronunciation. Parallel texts are short readings that help illustrate contrasts and similarities. The book aims to accelerate the work of third-language learning for students, travelers, and budding multilinguals.