## Mutual Object Positioning

The Item [Tangency](./Tangency.md) shows an example of tangency of a
circle and a line that can have two mutual positioning variants (Fig.
29.10.1). C3D Solver remembers the mutual orientation when it is created
and strives to maintain it during all further changes. A similar
selection can be enabled for linear and angular dimensions. For example,
a linear dimension for a point and a line presumes that the point would
always stay on the same side of the line (relative to the normal) if the
sign remains the same.

Mutual position of objects can be kept for such constraints, as
tangency, linear and angular dimensions. Other constraints, such as
parallelism, perpendicularity, collinearity, etc., permit to change
mutual positions. For example, perpendicularity of two lines may
transform a left vector pair into a right vector pair by changing their
mutual position. Therefore, the angle between their directions can be
either 90 or 270 degrees.