You should purchase an insole that matches the size and width of the shoe that the insole will be used in. If your shoes are a narrow width, you should purchase a narrow insole to ensure a proper fit of the insole inside your shoe. Placing a regular-width insole inside a narrow-width shoe could cause the sides of the insole to buckle or fold when placed inside the shoe, or may result in the insole not fitting inside the shoe at all.