The article offers firstly a comparison, using a historical-critical method, of the double grade of jurisdiction in Italian and Spanish civil law. In relation to foundations of the double grade, we underline the following considerations which flow from the arguments for and against the same: the attempt to revalue the role of the court of first instance, without interpreting in an authoritarian way the hierarchy of the courts; the real desire that citizens should be able to obtain the justice they seek from the courts without undue delay; the wish to afford everyone the possibility of appeal against a decision which they consider unjust, given that every citizen has a right to self-defence found in the constitution. In the legal systems of the state, even though they accept the favor veritatis, greater space and importance is attributed to the principle of the certainty of law. Secondly, the article offers a study on the origin and nature of the procedural principle in canon law of the double conforming sentence, wondering if it really answers the desire to produce a judgement consistent with the favor veritatis. The article goes on to illustrate the links that the double conforming sentence may have with the favor matrimonii and with the need for moral certainty. The subjectivity of the second instance judges may play great importance in determining the substantial truth, which leads us to analyse the very concept of conformity of sentences, thus arriving from this analysis at the assumption of substantial or equivalent conformity.