What is the difference between the overall length and the center-to-center of a cabinet pull? Where can I find this information?

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Overall length is the total length of the pull. Center-to-center is the distance between screw holes on the pull body. The center-to-center distance is how you determine what size pull you need to swap out existing cabinet hardware. 
This information can usually be found in the image carousel (the collection of images on a product page). It might also be listed as product dimension length with other details such as finish and material. Lastly, you can find the overall length in the specifications section on the product page.