Shampoos have a pH of between 4 and 6 and do not contain soap. Soapless shampoos are acidic and therefore closer to the natural pH of hair. Acidic shampoos are the most common type used and maintain or improve the condition of the hair as they don't swell the hairshaft and don't strip the natural oils. Conditioners are often used after shampooing to smooth down the cuticle layer of the hair, which can become roughened during the physical process of shampooing. There are three main types of conditioners: anti-oxidant conditioners, which are mainly used in salons after chemical services and prevent creeping oxidation; internal conditioners, which enter into the cortex of the hair and help improve the hair's internal condition (also known as treatments); and external conditioners, or everyday conditioners, which smooth down the cuticle layer, making the hair shiny, combable and smooth. Conditioners can also provide a physical layer of protection for the hair against physical and environmental damage.