Which 2D constraint can be used to align two linear elements?

The collinear constraint is specifically designed to align two linear elements along the same line. When this constraint is applied, it ensures that the two lines share the same direction and lie on top of one another. This is particularly useful in 2D sketching environments where proper alignment is crucial for accuracy and functionality in design.

In contrast, while parallel constraints align two lines in such a way that they never intersect, they do not ensure the lines are on the same path or share the same point. Horizontal and vertical constraints, on the other hand, restrict the orientation of the lines to those specific directions, which might not result in alignment if the lines were initially at different angles. Therefore, for directly aligning two lines, the collinear constraint is the most effective choice.