Take notice of this one, especially if you have shorter or longer legs, most jeans can be turned up or re- hemmed, but for off the shelf perfect fit get your inside leg right.
Hipsters or High Waist? It's all about this measurement. Basically the join or seam in crutch to top of waist- If you know where you like your jeans to sit this measurement will help find your perfect pair.
How much stretch do you like? or maybe you like 100% cotton no stretch denim, which is the most hardwearing. Some denim can blend up to 4% lycra/elastane. With 1 or 2% being most common. The more stretch the softer they are but they also won't last as long due to the nature of the material blend and should be cared for more delicately. If it has stretch it should be worn fitted.