There are a number of grades of commercially pure (unalloyed) titanium as identified in ASTM B265 (Gr1, Gr2, Gr3, Gr4, Gr7, Gr11, and Gr12). Each grade has a different amount of impurity content (Grade 1 has lowest impurities). Grade 1 to 4 is classified as pure even though grade 4 is much stronger and less ductile than grade 1. Grade 4 contains higher levels of oxygen which is classified (for pure titanium) as an alloying element. Oxygen and Nitrogen and Carbon are all interstitial alloys (they insert themselves in the crystal interstices of the crystals and prevent relative sliding). They therefore increase the hardness and reduce the ductility. Oxygen is the main element controlling the strength of unalloyed titanium.