The seven categories are as follows:

Gingivitis
Chronic periodontitis
Aggressive periodontitis
Periodontitis as a manifestation of systemic disease
Necrotizing ulcerative gingivitis/periodontitis
Abscesses of the periodontium
Combined periodontic-endodontic lesionsMoreover, terminology expressing both the extent and severity of periodontal diseases are appended to the terms above to denote the specific diagnosis of a particular person or group of people. Severity
The "severity" of disease refers to the amount of periodontal ligament fibers that have been lost, termed "clinical attachment loss". According to the 1999 classification, the severity of chronic periodontitis is graded as follows:
Slight: 1–2 mm (0.039–0.079 in) of attachment loss
Moderate: 3–4 mm (0.12–0.16 in) of attachment loss
Severe: ≥ 5 mm (0.20 in) of attachment loss

Extent
The "extent" of disease refers to the proportion of the dentition affected by the disease in terms of percentage of sites. Sites are defined as the positions at which probing measurements are taken around each tooth and, generally, six probing sites around each tooth are recorded, as follows:

Mesiobuccal
Mid-buccal
Distobuccal
Mesiolingual
Mid-lingual
DistolingualIf up to 30% of sites in the mouth are affected, the manifestation is classified as "localized"; for more than 30%, the term "generalized" is used. 2017 classification
The 2017 classification of periodontal diseases is as follows:Periodontal health, gingival disease and conditions

Periodontal health and gingival health
Clinical gingival health on an intact periodontium
Clinical gingival health on an intact periodontium
Stable periodontitis
Non periodontitis person
Gingivitis - Dental biofilm induced
Associated with the dental biofilm alone
Mediated by systemic and local risk factors
Drug induced gingival enlargement.
On returning downward and possibly inward to take up fixation, the DVD slow movement will be reversed. The dissociative movement seen objectively should not be confused with the dissociation that occurs subjectively - as when the brain begins to not visualise both images simultaneously (by ignoring or suppressing vision in that eye). Diagnosis
A test called the Bielschowsky Darkening Wedge Test can be used to reveal and diagnose the presence of dissociated vertical deviation, although any (or no) amount of dissociative occlusion may also prompt it to occur. The patient is asked to look at a light. One eye is covered and a filter is placed in front of the other eye. The density or opacity of this filter is gradually increased, and the behaviour of the eye under the cover (not of the eye beneath the filter) is observed. Initially, if DVD is present, the covered eye will have elevated, but as the filter opacity is increased the eye under the cover will gradually move downwards. This Bielschowsky phenomenon is present in over 50% of persons with prominent DVD, all the more if the DVD is asymmetric and amblyopia is present as well.The Bielschowsky phenomenon is also present in the horizontal plane in patients with prominent DHD (dissociated horizontal deviation). Differential diagnosis
DVD is often mistaken for over-action of the inferior oblique extra-ocular muscles. DVD can be revealed on ocular movement testing when one eye is occluded by the nose on lateral gaze. This eye will then elevate, simulating an inferior oblique over action.