In riding it is common to talk about the riders seat and that a well balanced rider is sitting with his/her body oriented around the vertical line, with ear-shoulder-hip and ankle nicely aligned.
Lena and I took some pictures where she is acting rider and I hold the (at times vertical) line. The first image below illustrates the now so common slightly backwards tilted seat - the rider's body differs greatly from the vertical.
The attentive reader can see that Lena is compensating for the backward leaning with advancing both the lower leg and poking her head forward to avoid tilting backwards. Sitting on the wooden horse she can keep herself balanced by tightening her muscles. On a horse in motion, she inevitably will use the reins to keep her balance.
You can get a fairly straight line between shoulder, hip and heel, just as we do here in the picture, but as it meet the surface of earth at an angle less than 90 degrees it may well be a straight but not a vertical line.
In this picture we show what is called a hanging seat (rough translation from Swedish..) Here you can see that the deviation from vertical is less than in the upper image and the angle is greater than 90 degrees to the ground. Even in this seat the rider can become unstable and have a need to balance herself in the bridle, but as long as the horse is checked, riders are able have a light contact in the bridle.
The last picture is our contribution to illustrate the vertical seat and a balanced rider.