There isn’t a single strict rulebook for framing photos; much of it is subjective and depends on the image and your taste. A useful term to research is art framing aesthetics.

Common guidance from framers and photographers:

• Matted prints often look more finished than prints placed directly in a frame.
• For black-and-white prints, simple white or black mats with matching or similarly neutral frames are common choices.
• For color prints, choose mat and frame colors that complement the image rather than compete with it.
• Keep the frame and mat visually balanced with the print size: large prints usually need more substantial mats/frames than small prints.
• A common rule of thumb is that the mat is often wider than the frame, sometimes around twice as wide.
• Avoid making frame width and mat width identical unless they are the same color.

Beyond that, presentation is highly aesthetic: thick vs. thin mat, thin vs. heavy frame, neutral vs. colored mat, and even unconventional mat proportions can all work if they support the image. The most practical approach is to view sample mats and frames beside the print in person and choose what best suits the photograph.