The appearance of horizontal lines on the nails may be caused by onychomycosis, psoriasis, onychomycosis and other diseases. 1. Onychomycosis: Onychomycosis is caused by dermatophyte infection, and patients may have cloudy nail plate, separation, atrophy, uneven surface, as well as changes in nail color, which may be yellow, dirty color, and manifested as nails with horizontal lines. 2. Psoriasis: Psoriasis may be caused by heredity, environment, pathogen infection, excessive pressure and other factors. Patients may have symptoms such as nail separation, hyperkeratosis underneath the nail, nail depression, etc., which may be manifested as nails with horizontal stripes. 3. Inguinitis: Inguinitis may be caused by localized infection, long-term contact with irritating substances, and being in a stuffy and humid environment. Patients’ nail morphology changes, manifested as grooves, discoloration, nail plate hypertrophy, etc., nails can appear horizontal lines. There are many reasons that lead to the appearance of horizontal lines on the patient’s nails, and they should go to the hospital in time to find out the cause of the disease.