What is 2-pass vs. 3-pass blockout curtains?

2-pass Blockout Curtains: These have two layers. The first layer is a coloured curtain fabric, and the second is a black foam coating on the back. This black layer is designed to block out light and improve insulation.

3-pass Blockout Curtains: These include an additional layer, making three total. There's the front decorative fabric, a black foam layer in the middle for light absorption, and a white foam layer on the back. This extra layer enhances the blackout properties, increases thermal insulation, and often improves the curtain's overall appearance from outside the window.