The line of regression is the line which give the best estimate to the values of one variable for any specific values of other variable.
For two variables on regression analysis there are two regression lines. One line as the regression of X and Y and the other for regression of Y and X , these two regression lines show the average relationship between the two variables. The regression line of Y on X gives the most probable values of Y for given values of X and the regression lien of X and Y gives the most probable values of X for given values of Y.
For perfect correlation positive or negative i ,e, for r= ± the two lines coincide i, e, we will find only one straight line. If r= 0 i, e, both the variance are independent then the two lines will cut each other are a right angle. In this case the two lines will be parallel to x and y axes. (see figure).