The acrylic two way mirror can be installed in either direction, so it can pass the fingernail test. Therefore, you will need other methods to detect a two way mirror.
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Acrylic Two Way Mirror Buy Precision Cut Acrylic 2-Way Mirrors
Use with MirrEdge acrylic mirror framing strips (sold separately) to frame existing bath and vanity mirrors without having to remove the mirror from the wall. Acrylic Mirror Corner Plates are applied one in each corner area on top of MirrEdge Framing Strips to complete framing the mirror. Corner Plates eliminates the need to miter framing strips in each corner area.
MirrEdge 32504 Acrylic Mirror Corner Plates
No space, leave this place'? Finger test won't always tell the
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