We present long-slit optical spectra along the major axis of the planetary nebula NGC 7009 .
These data allow us to discuss the physical , excitation and chemical properties of all the morphological components of the nebula , including its remarkable systems of knots and jets .
The main results of this analysis are the following : i ) the electron temperature throughout the nebula is remarkably constant , T _ { e } [ O iii ] = 10 200 K ; ii ) the bright inner rim and inner pair of knots have similar densities of N _ { e } \sim 6000 cm ^ { -3 } , whereas a much lower density of N _ { e } \sim 1500 cm ^ { -3 } is derived for the outer knots as well as for the jets ; iii ) all the regions ( rim , inner knots , jets and outer knots ) are mainly radiatively excited ; and iv ) 
there are no clear abundance changes across the nebula for He , O , Ne , or S. There is a marginal evidence for an overabundance of nitrogen in the outer knots ( ansae ) , but the inner ones ( caps ) and the rim have similar N/H values that are at variance with previous results .
Our data are compared to the predictions of theoretical models , from which we conclude that the knots at the head of the jets are not matter accumulated during the jet expansion through the circumstellar medium , neither can their origin be explained by the proposed HD or MHD interacting-wind models for the formation of jets/ansae , since the densities as well as the main excitation mechanisms of the knots , disagree with model predictions .