Knots hold countless things together, from the fabric in our clothes, to our cities’ skyscrapers. In recent decades, chemists have knotted tiny strands of molecules together in test tubes using a series of chemical reactions. But out of the 6 billion known prime knot formations—knots that can’t be created just by sticking multiple formations together—only three have been created in total: the trefoil, the figure-eight, and the pentafoil. Now, a new study depicts a fourth: the 819 knot. Though past knots have only been woven with two strands, the 819 is made with three, which researchers used a five-step chemical reaction to braid together. And at just 20 nanometers long and containing 192 atoms total, this new knot, described today in Science, crosses strands every 24 atoms to make the tightest physical knot ever. Researchers say it’s unclear what properties molecular knots could confer when applied to polymers and fabrics, but, using this new braiding method, they hope to uncover how knots might give myriad materials more strength and flexibility—from surgical sutures to bulletproof vests.