A more thorough explanation:

The common-source doctrine is a legal principle that applies to trespass-to-try-title actions. It states that if a defendant claims ownership of a property under the same source as the plaintiff, they cannot prove their title using a third source that is superior to the common source. This is because doing so would be considered an attack on the source under which the defendant claims ownership.

Let's say that John and Jane both claim ownership of a piece of land that was originally owned by their grandfather. John inherited the land from his father, who inherited it from his grandfather. Jane, on the other hand, inherited the land from her mother, who also inherited it from their grandfather. In this case, the common source of ownership is their grandfather.

If John were to claim that he has a superior title to the land because he found a deed that shows his grandfather sold the land to someone else before he died, this would be an attack on the source of ownership that he shares with Jane. Therefore, the common-source doctrine would prevent John from using this deed to prove his ownership of the land.

Another example could be if two siblings inherit a property from their parents. One sibling cannot claim ownership of the property by showing a will that gives them the property, as this would be an attack on the common source of ownership, which is the parents.