Lewis F. Day (1845-1910) was a designer, writer, and critic who published several books that informed readers of patterns with which they might not otherwise have been familiar. These patterns covered a wide range of styles but noteworthy are the geometric patterns that involved diagonal relationships. This is critical because when Day's books came on the market, it was when diagonality as a design motif in the modern sense of the word was just about to emerge.

Although no certain linkage is evident yet in my research, and while the diagonal motif was not the only kind of patterns Day illustrated, one can imagine that Day's images of diagonal patterns did influence the world of design , especially the work of Frank Lloyd Wright.

Day's titles include:

• The Anatomy of Pattern
• Ornament and Its Application 1904
• Nature and Ornament
• Pattern Design
• Lettering In Ornament
• Alphabets
• Some Principles of Every-Day Art
• Alphabets Old and New
• Moot Points: Friendly Disputes Upon Art and Industry with Walter Crane
• A Book About Stained Glass