Alignments

When the pointer snaps to one endpoint of an object, that location is stored as current origin of the temporary guides; the guides are displayed when the cursor is on the X-axis or Y-axis of that datum point. 

The pointer also snaps to the intersection of the guides: the intersection is highlighted with one point and both the guides visible.

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Furthermore, when you place the pointer over a segment, this function stores the angle of that segment displaying the corresponding parallel and orthogonal guides: when tracing the line, just place the pointer over one datum point of that segment (endpoints or midpoint) to display the alignment. 

To temporarily disable the Snap to alignments, hold the “cmd” key (Mac) or “ctrl” (Windows).