As part of a larger study (Boling et al., 2024a, b), this paper reports on how instructional designers define and use precedent knowledge, including its sources, forms, and role in decision-making. Using a qualitative approach, we conducted semi-structured interviews with instructional designers from higher education and non-profit professional settings to explore how precedent knowledge is built, stored, and applied in instructional design practice. The findings of the study indicate that instructional designers implicitly engage with precedent knowledge, drawing on personal experiences, established frameworks, and external influences such as professional networks. They use precedent knowledge for validating design choices, modifying existing designs, and inspiring new projects, often as part of collective-held schema (e.g., design models, such as ADDIE and SAM). The study highlights the underrecognized role of precedent knowledge in decision-making within the instructional design practice, emphasizing the need for greater awareness and integration of precedent knowledge in instructional design education and professional development.