Everything above,, its a combination of the material itself and obviously hair covering it.. the color of the lace, how it matches your skin, how translucent it is, etc.. the pattern that allows your own skin to show through.. the light that reflects off your skin that helps to hide the threads because of the way light bounces back. That is why lace looks better right dead on with light above you, as opposed to off lighting conditions where there is no light being bounced back off your skin and lace.