The rule you're referring to is Chargaff's Rule. It states that in the DNA of any species, the amount of Adenine (A) is equal to the amount of Thymine (T), and the amount of Guanine (G) is equal to the amount of Cytosine (C).

This is because DNA forms a double helix where A always pairs with T, and G always pairs with C. This pairing is due to the specific hydrogen bonding between the nitrogenous bases.