49Moreover, under Article 139 of the detailed implementing rules, the contracting authority is obliged to allow the tenderer to clarify, or even explain, the characteristics of its tender before rejecting it, if it considers that a tender is abnormally low. The obligation to check the seriousness of a tender also arises where there are doubts beforehand as to its reliability, also bearing in mind that the main purpose of that article is to enable a tenderer not to be excluded from the procedure without having had an opportunity to explain the terms of its tender which appears abnormally low.