Since cot θ = O when θ = 90°, the fractional error in d (i.e., l:ldld) is zero when θ = 90°. However, since we cannot observe an x-ray reflection at θ = 90° (this will be the back-reflected beam), we should consider reflections as close to the value of θ = 90° as possible to get the best precision and calculate the true value of the lattice parameter(s) as θ approaches 90°. Unfortunately, the plot of lattice parameter versus θ is not linear; therefore, extrapolation of the lattice parameter versus θ curve to θ = 90° does not lead to accurate results. However, extrapolation of the lattice parameter(s) against certain functions of θ will produce a straight line, which can then be extrapolated to the value corresponding to θ = 90°. Also note that the exact nature of this extrapolation function depends on the kind of equipment used to record the x-ray diffraction pattern (type of camera or diffractometer), as we will show.