Ok, here is my problem. I haven't taken anything vector related since at least one year ago. And back then, I wasn't such a good student.. So now my past has come back to haunt me.. I still have some basic notions, but other than that, I pretty much forgot things.. http://img393.imageshack.us/img393/8378/mec1wi0.jpg [Broken] Can someone please explain to me why ir = cosθ . i + sinθ . j and iθ = -sinθ.i + cosθ. j ? Why isn't for example iθ = -cosθ.i + sinθ.j? Our teacher told us that these two are formulas. Or do they vary in each case? (ir, iθ, i and j are vectors.) Thanks in advance for your help.