Eighteenth-century Spanish keyboard music reflects a blend of native traditions and external influences, notably the Italian Galant style. Central to this influence is the partimento tradition -- a pedagogical method emphasizing harmonic practice and improvisation, originating in Italy around 1700. Despite its importance, the presence of partimento in Spain has been underexplored. This paper investigates the only known work in the partimento tradition by a Spanish composer: Reglas Generales, o Escuela de Acompañar de Órgano o Clave (c. 1780s), an unpublished manuscript by Félix Máximo López, fourth organist of the Royal Chapel in Madrid. Reglas Generales is concise in its language but abundant in musical examples and exercises, reveals a sophisticated understanding of partimento pedagogy, and offers a clearer window into harmonic practice than modern methods typically allow. The findings not only enrich our understanding of eighteenth-century Spanish music but also invite a broader perspective within historically-informed analysis.