The binormal probability density function

PDF[MultinormalDistribution[{20,10},{{25,9},{9,9}}],{x,y}]

ContourPlot[ PDF[MultinormalDistribution[{20,10},{{25,9},{9,9}}],{x,y}],{x,10,30},{y,5,15}]

Manipulate[
 Module[{v,normalization},v={{σx^2,ρ σx σy},{ρ σx σy,σy^2}};
normalization=1/(2Pi Sqrt[ Det[v] ]);ContourPlot[ PDF[MultinormalDistribution[{μx,μy},v],{x,y}],{x,μx-3 σx,μx+3 σx},{y,μy-3 σy,μy+3 σy},Contours->normalization Range[9]/10]],
{{μx,20},0,50},{{μy,10},0,50},
{{σx,4},3,5},{{σy,3},2,4},
Style["Notice that note only ρ, but also σx and σy affect the inclination of main axes - keep an eye on the coordinate scales",12],
{{ρ,0.6},-0.9,0.9,Appearance->"Labeled"}]