As well as being the longest tooth in the mouth, its oval canal often seems very spacious during instrumentation. However, there is usually a sudden narrowing at the apical 2 3 mm; this leads to a danger of overinstrumentation if too large a file is used at this level. The length of this tooth can be difficult to determine on radiographs, as the apex tends to curve labially and the tooth will appear to be shorter than it actually is. The oval shape of the root canal is reflected in the shape of the access cavity.