Match Seam

You can use the Match Seam tool when you want to place one piece over another piece in order to compare and replicate a seamed segment.

Icon & Location

To use the Match Seam tool:

Select the Match Seam tool.
Select a corner point on the stationary piece.
To select the previous segment (counter clock-wise), hold the Shift while selecting the corner point.
Select the matching corner point on the second piece to move piece on piece.

The non-stationary piece appears on top of the stationary piece and the Move Piece on Piece dialog appears as follows:
In the Aligned Direction area, select one of the following options:

To Previous Point: If you want the two segment that come before the two selected clockwise points to be attached.

To Next Point: If you want the two segments that continue the two selected clockwise points to be attached (This option is the default option).
Use the Zoom In/Zoom Out buttons to zoom in or out to get a closer look at the seam.
Clear the Open this dialog without Alt button checkbox, if you want this dialog to appear when you press the Alt button when you are selecting two points.
Select the Flip Moving Piece checkbox, if you want the moving piece to flip over and be put on the stationary piece as sewn (in most cases).
Click Apply when you are done.
Select the first intersection point of the two seams (clock-wise):
Select the second intersection point of the seams.
The seam will replicate according to the selected piece.
Click the Esc button to exit the tool and place the moving piece back to its original position.

The piece with the replicated seam will get a Custom Seam Segment setting.