**Core Concept**
Miller's classification of increased tooth mobility is a clinical grading system used to assess the degree of tooth movement in response to various factors, including periodontal disease. It helps dentists and periodontists to evaluate the severity of periodontal disease and plan appropriate treatment. The classification system grades tooth mobility from grade 0 (no mobility) to grade 4 (maximal mobility).

**Why the Correct Answer is Right**
Tooth mobility in the horizontal direction is a common indicator of periodontal disease, but Miller's classification also considers vertical mobility. Grade 3 mobility in Miller's classification is characterized by a significant increase in both horizontal and vertical tooth movement, indicating advanced periodontal disease. This grade of mobility is often associated with the loss of periodontal support and attachment, which can compromise the long-term stability of the tooth. The presence of both horizontal and vertical mobility in grade 3 suggests that the periodontal disease has progressed to a stage where the tooth is at risk of being lost.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because grade 3 mobility in Miller's classification is characterized by significant tooth movement, which is more than 0.1 to 0.2 mm in the horizontal direction. This option represents a more minor degree of mobility.
**Option B:** This option is incorrect because grade 3 mobility in Miller's classification involves more than just 1 mm of horizontal tooth movement. The presence of vertical mobility is also a key characteristic of this grade.

**Clinical Pearl / High-Yield Fact**
Miller's classification of increased tooth mobility is an important tool for periodontal diagnosis and treatment planning. Remember that grade 3 mobility is characterized by significant tooth movement in both the horizontal and vertical directions, which indicates advanced periodontal disease.

**✓ Correct Answer: D. Mobility in horizontal and vertical direction**