A more thorough explanation:

Definition: Ad communem legem (ad kəˈmyo͞onəm ˈlējəm), n. [Law Latin “to common law”] is a historical writ of entry that was available after the death of a life tenant to recover a reversionary interest in land alienated by the tenant.

Example: If John owned a piece of land and granted a life estate to his friend, Jane, then upon Jane's death, the land would revert back to John. However, if Jane sold the land to someone else during her lifetime, John could use the writ of entry ad communem legem to recover his reversionary interest in the land.

This example illustrates how the writ of entry ad communem legem was used to protect the interests of the original landowner, John, after the death of the life tenant, Jane.