Ancestry launched ThruLines in the spring of 2019. Initially, there were some glitches, but overall their tool was useful. Since then, they have made improvements, thereby increasing the usefulness of this tool.
Go to “Trees,” “Create and Manage Trees,” and then click, “Create a New Tree.” Ancestry will begin prompting you to add yourself, your parents, etc. You can save and name your tree after adding three people. For ThruLines to work, you must go back four generations. When researching the people in your tree, be careful using the hints provided by Ancestry. Take a close look at and verify the information. Bad habits are hard to break. Taking someone else’s research as true can be a costly mistake down the road. Read and verify everything! Set a goal to have 4-5 generations, well documented, in your tree. By the time your DNA is complete, you will have a solid foundation for researching. 
Scroll down to “Privacy,” and under “Display Settings,” make sure you have selected, “You can see your DNA matches, and you’re listed as a match.” Your selection will automatically save. 
You need to link your DNA with ‘you” in your family tree. After logging into Ancestry.com, look at the top toolbar and click on “DNA,” then click on “DNA Results Summary.” Your “Results Summary” screen will open. Next, click on “Settings,” which will be on the right side of the screen. Go down to “Tree Link,” and you will see “this tree is linked to,” click change. Your family tree list will open. Select your family tree and select yourself.
To access Thrulines, click on “DNA,” and then click on “Your DNA Results Summary.” It takes about 48 hours for ThruLines to activate once you connect your DNA to your tree. 
For years I have spent countless hours trying to find the common ancestor of my DNA matches, which was often a daunting task. ThruLines has made this much easier.
These are my great-grandparents (from my tree) listed in ThruLines. When hovering your mouse over a person, a caption displays indicating how many DNA matches they have found who also has this person in their tree. The caption for William McCurdy Cubbison, shows there are four matches. After hovering, click the mouse on the person, in this case, William McCurdy Cubbison, and the following screen opens up.
These are my four matches. One is my father, and the other three descend from Mareen (my grandfather’s unknown half-sister.) William McCurdy Cubbison had another family, and from that family, there are three descendants with whom I share DNA.
Be very careful to review all of the “potential ancestor” hints. If there is inaccurate information in your tree, you may receive inaccurate ThruLines information. If there is inaccurate information in someone else’s tree, you may receive inaccurate ThruLines suggestions.
For instance, my husband’s 4th great-grandfather was John Curtis (1800-1844). In the 1950s, Harlow D. Curtis compiled research and continued documenting this ancestral line in his book, A Genealogy of the Descendants of Henry Curtis of Sudbury Mass. In this book, John Curtis’ father (and several of his siblings born between 1813 and 1847) was said to have been Samuel Curtis III. Unfortunately, Samuel Curtis III died in 1810 (as evidenced by his gravestone below.) He couldn’t have been the father of John and his siblings. Samuel is one of four children born to Samuel Curtis and Eunice Flagg, who died at a young age and was buried together with his siblings in Hope Cemetery in Worchester, Massachusetts.
A Curtis genealogy research book had misinformation; therefore, hundreds of people have this erroneous information, and ThruLines suggests to me that John Curtis’ father is Samuel Curtis III because of this. Books are not always correct, and the most popular answer is not always correct. Samuel Curtis The Son of Eunice Flagg and Capt. Samuel Curtis: Documentation Errors
I applaud Ancestry as they expand their tools and keep their site competitive. Ancestry has resolved several glitches associated with this tool back in March of 2019 making it a reliable and handy tool. Ancestry, by far, has the best tools today to help people like me. Without Ancestry, the discoveries I have made for so many people would not have been possible.