The authoritative statement of the arm’s length principle as used in transfer pricing is found in paragraph 1 of Article 9 of the OECD Model Tax Convention, which forms the basis of bilateral tax treaties involving OECD member countries and an increasing number of non-member countries.
Article 9 provides: [Where] conditions are made or imposed between the two [associated] enterprises in their commercial or financial relations which differ from those which would be made between independent enterprises, then any profits which would, but for those conditions, have accrued to one of the enterprises, but, by reason of those conditions, have not so accrued, may be included in the profits of that enterprise and taxed accordingly.
The analysis required to apply the arm’s length principle on controlled transactions is referred to in the transfer pricing guidelines as a comparability analysis. See TPG 1.6.