The ideal orientation of retentive pin holes is parallel to the nearest external surface because this ensures maximum retention without compromising tooth structure. Drilling pins perpendicular to the external surface risks piercing the pulp or causing unnecessary weakening of the dentin. Parallel alignment follows the natural contour of the tooth, providing optimal resistance to dislodgement forces while preserving structural integrity. This orientation also facilitates easier insertion and better adaptation of the pin within the dentin. Maintaining parallelism reduces the chance of pin loosening and improves the longevity of the restoration.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.