In this paper, we present principles of pattern illustration design and summaries of subsequent pattern seeds. Pattern illustration is a visual expression to convey the core idea of a pattern symbolically, which is shown in combination with its pattern name. Although many patterns for drawing pattern illustration have been proposed, this paper focuses more on fundamental principles of pattern illustration design. The principles were extracted by analyzing more than 600 pattern illustrations for 19 pattern languages in various domains, which we have been creating for the past decade. Our discoveries on design principles of pattern illustration are grouped into the following 12 principles: (1) Space that has Depth; (2) Characters with a Mind and Body; (3) Interactions on the Spot; (4) Time Flow from Left to Right; (5) Symbolic Object for Abstract Things; (6) Visible Lines for Clusters and Connection; (7) The Effect Given by Things Created; (8) Object-like Speech Bubbles; (9) Visual Expressions in Comics; (10) Abbreviation for Simplicity; (11) Evolving Stories from Patterns; and (12) A Magical Touch for Charms. In this paper, we explain these principles, the subsequent pattern seeds, and examples from our pattern languages.