Abstract: A steel cord comprises a central bundle of three twisted steel filaments (1) with a diameter d.sub.1 and nine steel filaments twisted around said central bundle with the same twist pitch and twist direction as the central bundle. Three filaments (2) of said nine steel filaments have a diameter d.sub.2 and contact two adjacent steel filaments of the central bundle. Six steel filaments (3) of said nine steel filaments have a diameter d.sub.3 and contact both one steel filament of the central bundle and one steel filament of the three filaments with diameter d.sub.2. The ratio d.sub.2 /d.sub.1 is greater than 1.05 and smaller than 1.16. The raio d.sub.3 /d.sub.1 is greater than-0.205=0.814.times.d.sub.2 /d.sub.1 and smaller than-0.105+0.814=d.sub.2 /d.sub.1.