What is the cardboard inside a picture frame called?

The cardboard inside a picture frame is called a mat board. Picture frame mats are thick & add a professional touch to your framed images.
The thick material inside a picture frame is called a picture frame mat. It's typically sturdy like cardboard, but it isn't actually cardboard. Crafted out of sturdy mat board material, picture frame mats are an easy way to highlight a framed photo, art print, or map. They are available

Cardboard Types, Cardboard Fundamentals

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Picture Frames Parts Names - Photo Frames and Picture Frames Online Store

Frame Types

Picture Frames Parts Names - Photo Frames and Picture Frames Online Store

Matboard and More

Mat (picture framing) - Wikipedia

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