This European Telecommunication Standard (ETS) specifies rules for the use of the Specification and Description Language (SDL)@ defined in ITU-T Recommendation Z.100 [1] and Message Sequence Charts (MSC)@ defined in ITU-T Recommendation Z.120 [3]@ in ETSs. Object oriented extensions in SDL are not considered. It is intended that SDL and MSC diagrams will be used in ETSs@ in combination with text@ informal figures and tables. SDL diagrams are to be used to formalise those parts of an ETS that need to be defined precisely and unambiguously. Furthermore@ it is recommended to use the Abstract Syntax Notation One (ASN.1)@ formalism defined in ITU-T Recommendation X.680-X683 [7]@ that is related to SDL. Therefore@ this ETS also specifies rules for the use of ASN.1@ when used in combination with SDL. All rules are aimed at improving the possibilities to validate a standard in an early phase of development@ and to improve the possibilities to test products that claim to implement a standard. This ETS does not contain any changes to the (formal) semantics of SDL@ MSC or ASN.1. The use of SDL and related formalisms@ enforced by the rules in this ETS@ fully conforms to the language definitions of SDL@ with exception of the use of ASN.1 within SDL diagrams. This ETS does not provide a methodology for the production of ETSs which contain SDL. This ETS does not restrict the use of ASN.1 in ETSs@ if ASN.1 is used otherwise than in combination with SDL diagrams. This ETS is not aimed at the use of SDL for product design and development. The subset of SDL that is selected in this ETS is tailored to standardisation@ where clarity is of the utmost importance. This ETS is not applicable to the specification of conformance test standards.