Rule 1
Classification should follow, not precede, any extractions of teeth that might change the arch. The arch must be classified after extractions to reflect the final edentulous condition accurately.
Rule 2
If a third molar is missing and not planned to be replaced, it is excluded from the classification. It has no bearing on the arch design.
Rule 3
If a third molar is present and will serve as an abutment, it must be included in the classification since it affects the prosthesis design.
Rule 4
A missing second molar is not considered in the classification if it will not be replaced (e.g., when the opposing second molar is also missing and not replaced).
Rule 5
The most posterior edentulous area always determines the classification type (e.g., Class I, II, III, or IV).
Rule 6
Edentulous areas other than those that determine the classification are called modifications and are numbered accordingly (Modification 1, 2, etc.).
Rule 7
Only the number of modification spaces is counted — not their size or extent.
Rule 8
No modification spaces can exist in Class IV arches, because any additional edentulous space posterior to the anterior span would redefine the classification (per Rule 5).
Class I
Bilateral edentulous areas located posterior to the natural teeth.
Class II
A unilateral edentulous area located posterior to the remaining natural teeth.
Class III
A unilateral edentulous area with natural teeth remaining both anterior and posterior to it.
Class IV
A single, but bilateral (crossing the midline) edentulous area located anterior to the remaining natural teeth.
