  In the paper we discuss the B\"acklund transformation of the KdV equation
between solitons and solitons, between negatons and negatons, between positons
and positons, between rational solution and rational solution, and between
complexitons and complexitons. We investigate the conditions that Wronskian
entries satisfy for the bilinear B\"acklund transformation of the KdV equation.
By choosing suitable Wronskian entries and the parameter in the bilinear
B\"acklund transformation, we obtain transformations between many kinds of
solutions.
