Useful skew is very important concept in CTS. Let’s discuss this through an example.
In the above picture, we can see that the first path is having positive 15 ps of skew, second path is having negative 5 ps of skew and third is having positive 5 ps skew. Now, if we can see among these three paths, the negative path can borrow skew from the first positive 15ps path, then skew will be balanced between the paths. After borrowing the skew, we can see that in below picture the skew looks positive in number for all the three paths. This concept is called as useful skewing and the skew we borrowed is useful skew.