Symptoms of chronic periodontitis are periodontal pocket formation, gingival inflammation, periodontal attachment loss and alveolar bone resorption, which are categorized into mild, moderate and severe according to the above symptoms. Symptoms of mild chronic periodontitis are periodontal pockets ≤4mm, loss of attachment 1~2mm, gingival inflammation, bleeding on probing, X-ray showing alveolar bone resorption not more than 1/3 of the root length, and bad taste in the mouth. Symptoms of moderate chronic periodontitis are periodontal pockets ≤6mm, loss of attachment 3~4mm, bleeding on probing and possible pus overflow, teeth may be mildly loose, gums are inflamed, and x-rays show alveolar bone resorption horizontally exceeding 1/2 of the root length. Symptoms of severe chronic periodontitis are periodontal pockets >6mm, attachment loss ≥5mm, gingival inflammation is more obvious, and prone to periodontal abscess, root bifurcation lesions in many teeth, teeth are loose, and X-ray shows alveolar bone resorption more than 1/2 or even 2/3 of the root length. Late periodontitis will also appear concomitant lesions and symptoms, teeth tilted or shifted; deep periodontal pockets close to the root tip, prone to root bifurcation lesions or acute pulpitis; periodontal overflow of pus or gap is too large, resulting in food inclusions, easy to lead to halitosis; gingival recession when the roots are exposed, easy to lead to sensitivity to hot and cold stimuli, but also prone to cause root surface caries.