The invention relates to glass reinforcement yarns having a composition containing the following constituents within the predetermined following limits in weight percent: SiO2 62 - 72 % AI2O3 2 - 10 % CaO 7 - 20 % MgO 1 - 7 % Na2O + K2O + Li2O 10 - 14.5 % Li2O O - 2 % BaO + SrO + ZnO O - 4 % B2O3 O - 4 % F2 0 - 2 % As2O3 0 - 0.15 %. These yarns are made of an economical glass that provides an excellent balance between the mechanical properties comprising the tensile strength and the fibre forming conditions. The invention also relates to composites containing organic and/or inorganic material(s) and to the above glass yarns.