• The incident angle, w, is defined between the X-ray source and the sample.

• The diffracted angle, 2q, is defined between the incident beam and the detector angle.

• The incident angle w is always ½ of the detector angle 2q .

• In a q:2q instrument (e.g. Rigaku RU300), the tube is fixed, the sample rotates at q°/min and the detector rotates at 2q°/min.

• In a q:q instrument (e.g. PANalytical X'Pert Pro), the sample is fixed and the tube rotates at a rate -q°/min and the detector rotates at a rate of q°/min.