Hi, I'm reading this piece from George Cain & James Harod's multivariable calculus material. Section 4.3, which is about Torsion, says this: I don't understand how he deduces dB/ds is perpendicular to T? Where did I get lost? Following the paragraph, it seems to me that T and N plays quite the same role to B, then suddenly dB/ds is perpendicular to T. Please enlightent me. Many thanks..