Business card-sized references for Arduino and basic electronics. I hand these out at workshops along with a couple LEDs and a coin cell in a 3x4" Ziplock bag. By design, these are NOT exhaustive references. 80/20 principle...the most common and essential information in the least space. They're aimed at beginners and the goal wasn't to handle every case, but to plant seeds: "Oh yeah, those resistor stripes mean something" or "Oh yeah, more current needs bigger wire" -- follow-on study then takes place elsewhere. Case in point, 5-band resistors aren't even mentioned, too esoteric for early learning.