Some of the correlations between morphological and other physical properties of planetary nebulae (PNe) are reviewed. In particular, the finding that bipolar b PNe have more massive progenitors than the other morphological classes is discussed in detail. Earlier results are confirmed; including all the various sources of uncertainty, the Galactic distribution of objects indicate that bipolar planetary nebulae are formed by stars with initial masses > 1.3 M&#x2299 while elliptical e PNe by progenitors with masses < 1.3 M&#x2299. Recent results for the chemical abundances of b and e PNe and their orientation within the Galaxy are also presented. Finally, the key role in the discussion of the formation of b PNe played by detached binary systems such as symbiotic stars is briefly discussed.