Abstract: Unlike the actual text for the proposed Common European Sales Law (CESL), which is based on extensive preparatory work by academics, the regime for opting into the instrument, which is set out in the main text of the proposed regulation, is entirely of the European Commission's own making. The approach adopted by the Commission is innovative and generally convincing, although there is still room for improvement. Characterizing the optional instrument as a second national regime rather than as a 28th regime has the important advantage of neutralizing the effect of Article 6 Rome I in B2C contracts, which makes the instrument sufficiently self-standing to become an attractive alternative. At the same time, this choice is acceptable politically and constitutionally (Article 114(3) TFEU) as long as the level of consumer protection within the common sales law remains as high as it is in the current proposal. In business-to-business (B2B) contracts, it is misguided for the regulation to suggest, as it does in the proposal, that a choice for the CESL would imply ipso facto the opting out of the CISG, since the question of what amounts to an exclusion of the CISG is a matter of interpretation of the parties' intentions under the CISG alone. Having said that, in most cases, it will not be very difficult for a court to deduce an implicit opting out of the CISG from an explicit opting into the CESL. Nevertheless, especially in the case of a partial choice for the CESL, which the proposal allows for B2B, interpretative difficulties may arise. The duty for sellers in B2C contracts to draw the consumer's attention to the intended application of the CESL by providing the consumer with a Standard Information Notice (SIN) is not likely to bring consumers much extra protection while the proposed sanction will bring undue legal uncertainty and should, therefore, be reconsidered. It also sends the wrong message, as if this new European law were something dangerous that consumers have to be warned about. The terminological choices of the proposal are not always felicitous. For example, it is not clear what is meant by the expression that the parties 'may use' the law. Of course, the proposed CESL does not fit neatly with any of the received dichotomies of international versus national contracts, Member State versus Union law, direct versus indirect applicability, and substantive versus conflict-of-laws choice of law. In some respects, therefore, the CESL may have to be regarded as sui generis. This is not at all surprising in light of the fact that the European Union (EU) itself is a sui generis legal order. However, the European legislator should try to make it as clear and foreseeable as possible for contracting parties what exactly will be the consequences if they opt into the CESL.