Hex symbols
What you say about hex symbols, Bill, is accurate, but I would add that the most common design (which includes triangles) is a star, usually 5-pointed. I would be willing to bet that this traditional design is derived from the pentacle, or 5-pointed star, used by magicians (or those claiming to be magicians) in the middle ages for charms and conjurations. This star design also shows up very frequently in quilt patterns; there are a number of well-known, standardized, star patterns for quilts.

P.S. You hardly ever see hex signs anymore, except as parts of a tourist come-on. Indeed, you rarely see a barn painted red any more; apparently it isn't cost-effective.