and AC season
What Mark said 
Nearly all pleated filters are designed to be unidirectional also, many even have arrows on them for air flow direction.
I’ve seen many that were installed backwards and were nearly sucked into the fan housing… Just some food for thought Nick.
Yup. Graphic needs fixed.
As a courtesy to my clients, I always snap a shot of the filter with it’s size in the picture and put it in the reports for future reference if needed.
Your graphic shows one of two most common locations for a furnace air filter. The other one is in the furnace itself. The next most common location is inside a return air grill mounted on a wall typically in a living room or hallway. It’s generally good practice to look inside the furnace for a second filter where the manufacturer provides a filter slot. I find extremely dirty filters inside the furnace overlooked by the homeowner.