The expositions for a normal probability plot (aka normal quantile plot) (in which observed probabilities are plotted against theoretical probabilities, or sometimes the other way around, to get a rough check as to whether a set of data is normally distributed by checking linearity) are not too clear (to me). To make this easy to answer, I will put my doubts into four succinct questions: First, which one standardly goes on the vertical axis: the observed or theoretical values? Secondly, does one put probabilities, or the values, on the axes? If probabilities, are the observed probabilities just calculated as per a frequency table? Thirdly (and this depends on the answer to the previous question), are the scales linear, a concatenation of logarithmic scales, or what? Fourthly, how does one calculate the theoretical value that goes to a given observed value? Thanks for answering any (or all) of these.