This paper describes a grammar for Perforated Masonry Walls of contemporary Paraguayan brick architecture. In these walls, bricks are arranged to generate different patterns, utilizing a series of rules that define the formal relation between oblongs. The masonry walls selected for this study provide shade and natural ventilation, and are used to improve thermal comfort in buildings. The paper, first, presents an analytical study that extracts the underlying generative rules for pattern-making of these masonry walls illustrating the construction logic of the walls. Then, a generative exploration that demonstrates the possible uses of the developed grammar is shown.