英语诗中的韵脚

请问各位高手：一直弄不太懂英文诗中的韵脚，比如说一首诗歌的韵脚是aabb, 还有什么ababcc ，这些字母到底表示什么意思？怎样判断，最近要考文学史，要分析每首诗歌的格律，可就是不太懂，请各位帮帮忙，详解一下！

第一个出现的韵脚就是a，第二个就是b，以此类推，举个例子

Two roads diverged in a yellow wood, /ud/ a

And sorry I could not travel both /oth/ b

And be one traveler, long I stood /ud/ a

And looked down one as far as I could /ud/ a

To where it bent in the undergrowth; /oth/ b

Then took the other, as just as fair c

And having perhaps the better claim, d

Because it was grassy and wanted wear; c

Though as for that, the passing there c

Had worn them really about the same, d

整首诗我就不妨在这里了，韵律是 abaab cdccd efeef ghggh

不明白的欢迎提问