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What is Root canal anatomy and variation will be best known when one takes and explain in detail?
Often a single radiograph of a multirooted tooth made at the normal vertical and horizontal projection does not display all the roots. In these cases, when it is necessary to separate the roots on multirooted teeth, a second projection may be made.
The horizontal angulation is altered,

20 degrees mesially for maxillary premolars.
20 degrees mesially or distally for maxillary molars.
20 degrees distally for an oblique projection of mandibular molar roots.

Ref: ORAL RADIOLOGY Principles and Interpretation,Stuart C. White,Michael J. Pharoah Edition 7 page no 129