Pure white non-printed masks can be judged by the direction of the pleats, if it is three pleats then the direction of the pleats down is the outside, if it is four pleats then the middle piece that is convex to the outside after propping up is the outside. In addition, the top two folds are upward, and the bottom two folds are downward. You can also read through the color, dark is outside, light is inside, or through the print to judge, the print is generally outside, the word is positive. Finally, you can also distinguish the positive and negative side by the metal strip, the side with the metal strip protruding is the outside, and the side with the metal strip flat is the inside.