Competition among companies that produce complex or large product portfolios has created a need to use modularity strategies not only to flexibly manage technical complexity in a cost-effective manner but also to produce visually appealing products. This research aims to understand how the visual appearance of products is affected by modular product development strategies and creates coherent product brands. Thus, this study examines the intersection of design aesthetics, product portfolio management, product brand management, and design management. Specifically, this study aims to understand how such strategies constrain and generate possibilities when the industrial design process concerns itself with visual appearance. The main research approach has been qualitative multi-case methodology (Miles et al, 2014; Eisenhardt, 1989) and design theory building (Chakrabarti and Blessing, 2016) that collects data through interviews, experimentation, and theoretical studies based on findings in the literature. Sixteen face-to-face interviews were conducted with design vice presidents, senior designers, and senior design engineers at five Swedish manufacturers from the automotive, MedTech, consumer goods, commercial vehicles, and materials handling industries. This approach has resulted in the description of three theoretical models and a design method, product gist, for investigating prototypicality in a product category. Aesthetic flexibility reflects the requirement that under certain circumstances an industrial designer has to plan for future (as yet unknown) changes in a design. Each of the three theoretical models has a different focus: one model describes three ways manufacturing companies organise a strategic in-house design function; one model describes how design decisions are made on a general level through an intuitive and knowledge-based judgment process; and one model describes the strategies a manager needs to consider when developing an existing product portfolio and how the strategies influence industrial design practice. Understanding visual flexibility serves as a starting point for further investigations of how development strategies affect visual product design. This understanding provides industrial designers insight into how they can develop product systems that share design components across product lines to promote brand identity. The findings of this work illustrate and explain a complex and multi-facetted design phenomenon that many designers manage more or less intuitively today; therefore, this study advances the understanding of the field for academics, teachers, and professional designers.