In the data set, you can see that when parents 170_1 and 171_11 were crossed, the progeny’s cross field level is set to 170_1x171_11, irrespective of which parent was used as female. The variable ‘r’ is the covariate that distinguishes the two reciprocals of this one combination, it is set at -1 when 171_11 was used as female and set at 1 when 170_1 was used as female. It is arbitrary which reciprocal is labeled as r = -1 or 1 within a particular combination. For example, when 170_1 is used as a female in a cross with parent 167_6, the progeny value for r is set at -1.