Hi, There are some points I really want to clear up in this topic....I promise to finish my chain of doubts as quicly as possible! I'll put in my first questions.... 1. Rotoinversion is a combination of inversion and rotation-- often it ends up as having the same effect on the crystal as reflection or just inversion--why,then is it recognized as a separate symmetry element? I could also put my question as 'what is the 'physical significance of rotoinversion,' since I read that all the symmetry elements have important roles in determining the chemical or physical properties of a crystal'-what is it in case of rotoinversion? 2. Inversion through a centre is also a symmetry element -- it is said that an object having one centre of inversion has one 'roto-inversion axis'--what is the relation between the centre of symmetry and 'rotoinversion'--why are they defined in terms of eachother? Can an object have more than one centre of symmetry? 3. Why is it not possible for a crystal to have a 5- or 8- fold rotational symmetry axis? 4. Again,translational symmetry elements must have some significance in determining the properties of crystals--firstly,why are they of only two types,screw and glide axes--couldn't we produce any other kind-like rotation followed by inversion? (I read that "A dislocation may glide only in the plane which contains both its line and its Burgers vector" - Is this the significance of 'glide' in crystal dislocations?) Thanks in advance for your help!