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A layer is simply one image stacked on top of another. A layer mask is something that you apply to a given layer to control the transparency of that layer. A mask gives more precise control over very specific areas of the layer over layer opacity which controls the transparency of the entire layer immediately.

Learn more about Layer Mask:

Photoshop For Beginners: Understanding Layer Masks by WeGraphics

How to Use the Layer and Clipping Masks in Photoshop by ClippingPathService

See also:

How To Use Photoshop Layer Mask by Guru99

Masking

Free Online Layer Mask Tools:

Lunapic

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