A new precedent set by the Spanish Supreme Court (known as the Parot doctrine) changed the method of applying remissions of sentences for work done in detention, postponing the date of final release of numerous inmates by several years. Retroactive application of such precedent has been held a violation of article 5 and article 7 of the Convention by the European Court of Human Rights (ECtHR). This paper provides some keys in that regard to understanding both the Parot doctrine and related ECtHR Judgments.