There exists a mucogingival junction on all three gingival surfaces on which there exists freely moveable alveolar mucosa: the facial gingiva of the maxilla and both the facial and lingual gingiva of the mandible. The palatal gingiva of the maxilla is continuous with the tissue of the palate, which is bound down to the palatal bones. Because the palate is devoid of freely moveable alveolar mucosa, there is no mucogingival junction.
The width of attached tissue is critical, because the more there is available provides a greater sense of protection against the aforementioned insults to the tissue. Using the mucogingival junction as the boundary demarcating the apical border of the attached gingiva, a periodontal probe in inserted into the gingival sulcus to measure how much of the keratinized gingiva coronal to the mucogingival junction is in fact attached to the underlying bone. The depth of the gingival sulcus, determined by the depth to which the probe enters the sulcus, is not attached to the underlying bone, and is subtracted from the total height of the keratinized tissue.
Thus, if the entire height of the keratinized gingiva, from the free gingival margin to the mucogingival junction is 8 mm, and the probing depth on the tooth at that location is 2 mm, the effective width of attached gingiva is 6 mm.
If the probe enters the sulcus and can descend up to or beyond the mucogingival junction, that area is said to represent a mucogingival defect.