Halakhah for Yevamot 41:20

From the laws of the commandment is that which they, may their memory be blessed, said (Sanhedrin 53a) that the wife of his father is forbidden by Torah writ, whether [she is his wife by virtue of] betrothal - the understanding of which is kiddushin (designation) - or from marriage; whether in the lifetime of his father and even if he divorced her, or after his death. And so [too,] that which they forbade (Yevamot 21a) the wife of his father's father - meaning to say the wife of his grandfather - even though she is not his grandmother, to make a fence for this prohibition. And so [too,] did they also forbid the wife of the father of his grandfather and so [too,] the wife of the father of the mother of his grandfather, and likewise going way back, even to Yaakov. And they also forbade just the wife of the father of his mother, as a fence for this prohibition. And the reason is because the essence of this prohibition is from the side of the father - meaning the wife of the father. Therefore they, may their memory be blessed, were more stringent with the side of the fathers and said that it be forbidden going way back forever; but from the side of the mother they only decreed just with the wife of the father of the mother. And this is similar to the distinction that we wrote above with the prohibition of the mother. And the rest of its details are in the places that we will write below (Sefer HaChinukh 211) at the end of the Order (see Tur, Even HaEzer 21). And this is from the sexual prohibitions that all people of the world were prohibited more generally. But there is a distinction between Israel and the rest of the nations - as with the rest of the nations, [one] is only called the wife of the father by way of intercourse, but with Israel, even by way of designation; and likewise regarding the secondary ones, as there is no prohibition of the secondary ones among the nations, as we said.

From the laws of the commandment is that which they, may their memory be blessed, forbade (Yevamot 21a) the daughter of the son of her son as a fence for this - and it is included in the secondary [prohibitions]. As everything the prohibition of which is from the words of the [Rabbis] is called a secondary prohibition. And so [too,] did they forbid only the mother of the mother of the father of his wife.