When determining the rhyme scheme of a poem, the first line is assigned the letter A. If the next line rhymes, it also receives the letter A. However, if the next line does not rhyme then it receives the next letter in the alphabet (B). You continue this pattern, moving through the letters of the alphabet, until you reach the end of the poem.
Therefore, the rhyme scheme of "Virtue" is ABAB CBCB DBDB EFEF. All of the even lines rhyme with each other until you reach the final stanza of the poem. The final stanza of the poem does not fit the rhyming pattern that the rest of the poem has followed. This is important because this last stanza delivers the final message of the poem. Just like the rhyme scheme in the final stanza is different, the poem itself has reached a turning point. In the first three stanzas of the poem, the author describes how everything must eventually die or come to an end. Stanza one describes the end of a day, stanza two describes a rose dying, and stanza three describes the end of spring. However, in the final stanza, the author expresses that "only a sweet and virtuous soul" (line 13) does not die. Unlike everything else that the author has discussed in the poem, a virtuous soul can have eternal life even after its earthly body has died. This change in expectation and the delivery of the final message changes the rhyme scheme of the poem so that the final message has a stronger impact.