A

The traditional outline for a mandibular first-molar access cavity is trapezoidal, allowing straight-line entry to the two mesial canals and the larger distal canal(s) and, when present, a distolingual canal or radix entomolaris; multiple anatomic studies describe widening the outline to a trapezoid to locate all orifices [Abella, 2012, PMID 22681628; De Moor, 2004, PMID 15479262].