The present invention relates to a liquid-crystal medium which comprises a component A which consists of one or more compounds of the formula X in which the parameters have the respective meanings given in the claims or in the text, and to the corresponding, novel mesogenic compounds and to the preparation thereof. The present invention likewise relates to the use of these liquid-crystal media, in particular in components for high-frequency technology, and to components of this type which contain the media according to the invention, and to the production and use of these components. The components according to the invention are suitable, in particular, for phase shifters in the microwave and millimetre wave region, for microwave and millimetre wave array antennae and very particularly for so-called tuneable “reflectarrays”.