by Julieanne Kost
Layer masks in Photoshop are white by default, they are not transparent—even though they control the opacity/transparency of the contents of the layer.
If you Option + (Mac) | Alt + (Win) -click on the layer mask thumbnail in the layers panel, you can view the contents of the mask. You will not see the checkerboard which, in Photoshop, represents transparency. Instead you will see white, black, or any shade of gray.
Where it might seem confusing is that the result of the mask is to show or hide the contents of the layer – in essence, hiding/revealing varying levels of transparency. But this is achieved by painting with shades of gray in the mask, not transparency.