Introduction:
The analysis of the literature and the results of my research and experience showed the need to search for biomechanical criteria that will make it possible to describe the body shape, especially in a sitting position, and will facilitate the implementation of therapeutic measures.

Aim:
Application of Euclidean geometry in the body posture assessment.

Material and methods:
The study uses Euclidean geometry and the concept of common sense to define mutual relationship of the position of the body of the sternum, the sacrum, thoracic kyphosis and lumbar lordosis. The findings are the result of long-time population observations.

Results and discussion:
The criteria found in the process of my clinical observations and studies allowed me to show close relationship of simultaneous movements and positions of individual parts of the body, i.e. the body of the sternum and the sacrum, as well as thoracic kyphosis and lumbar lordosis in relation to each other. On the basis of Euclidean geometry, this relationship was equated with the shape of a triangle, showing that common relationship between the specified body parts, the so-called ‘common sense,’ could be a method of assessing the body posture in a sitting position.

Conclusions:
(1) The movement of the sternum and sacrum system as well as a thoracic and lumbar spine are biomechanically correlated with each other. (2) Positioning the sternum to the value of the angle α may be a tip on how to control the body posture.