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(1) A midline of a triangle is the line segment joining the midpoints of two sides of the triangle. The midline is parallel to the third side, or base, of the triangle. The length of the midline is one-half that of the base. The areas of the two parts into which the midline divides the triangle are in the ratio 1:3.

(2) The midline, or median, of a trapezoid is the line segment joining the midpoints of the nonparallel sides of the trapezoid. The midline is parallel to the bases of the trapezoid, and its length is one-half the sum of the lengths of the bases.

References in periodicals archive ?
Maxillary midline diastema is usually normal after the eruption of maxillary central incisors.
The appliance presented in this case report is aesthetic, convenient for the patient, and sufficient to close maxillary midline diastema, particularly when the patient does not want fixed orthodontic treatment.
The midline diastema: a review of its etiology and treatment.
2-7 Etiology of non-coincident dental midlines include premature loss of one primary canine, crowding and missing teeth.
Correction of asymmetric molar and canine relationship as well as making non-coincident dental midlines coincident with each other and with facial midline is always challenging as differential mechanics are always required to achieve desired results.
The midline was centered on the middle of the remaining central incisor.
The objective of the present study was to determine the role of facial mid line as a guide for positioning maxillary dental midline for Prosthodontic patients during fabrication of prosthesis.
However patients having midline diastema orthodontic treatment or any cranio-facial anomaly were excluded.
For an adult patient with a severe skeletal Class III malocclu- sion and a midline deviation, combined surgical-orth- odontic therapy is often the treatment of choice be- cause of its satisfying outcome and stability.
Maxillary midline had shifted 3mm to the right and mandibular midline was shifted 3mm to the left side.