The Straighten Previous Pipes command is used to make pipes straight relative to the other pipes to which they are connected. The first pipe you select will be treated as straight by the command. All of the pipes that point to the selected pipe will be moved so that they are either parallel, perpendicular, or at a 45-degree angle to the selected pipe.
Any pipes connected to the pipes that are moved will have their ending points moved to stay connected. Their starting points will not be moved, which can result in pipes that are no longer straight. Continue using the Straighten Previous Pipes command until all of the pipes are straight.
To straighten pipes, go to
Ribbon: DM Plumbing->Pipes-> 
Straighten Previous Pipes
Pulldown Menu: DM Plumbing->Pipes->Straighten Previous Pipes
You will be prompted to select the pipe that is straight.
Select pipe that is currently straight:
The previous pipes of the selected pipe will be made straight.
See the Previous and Next Pipes section for more information about identifying the previous pipes.