The relationship between gauges and needle size. It's a simple concept, yet one that can mess with your head when you're a new nursing student. Especially during your few exams or clinical skills checkoffs (Sim Labs)
For those who don't remember Latin roots, here are some common medical prefixes and postfixes. Designed for doctors/nurses in training, but since many of us have to make our own medical decisions, it could be useful for anyone.