This part of IEC 61757 defines the terminology, structure, and measurement methods of distributed fibre optic sensors for absolute strain measurements based on spectral correlation analysis of Rayleigh backscattering signatures in single-mode fibres, where the fibre is the distributed strain measurement element in a measurement range from about 10 m to tens of km. The document is also applicable to hybrid sensor systems that combine the advantages of Brillouin and Rayleigh backscattering effects to obtain optimal measurement quality. This document also specifies the most important features and performance parameters of these distributed fibre optic strain sensors and defines procedures for measuring these features and parameters. This part of IEC 61757 does not apply to point measurements or to dynamic strain measurements. Distributed strain measurements using Brillouin scattering in single-mode fibres are covered in IEC 61757-1-2. The most relevant applications of this strain measurement technique are listed in informative Annex A, while informative Annex B provides a short description of the underlying measurement principle.