A photo pair is a before photo and an after photo taken from the same spot, shown side by side.
It's the quickest proof of what a crew did: the cracked lot and the sealed one, the faded stalls and the fresh stripes.
The best pairs are planned. Shoot the before from a spot you can find again, and stand there for the after. A caption of a few words, like "Front lot, cracks filled and sealed," tells the next person what they're looking at.
Ask before you put a customer's lot on your website. A pair that goes in a gallery should have the customer's say-so.