Ættbálkur er flokkunarfræðilegt hugtak sem á við um hóp dýra sem öll tilheyra sama flokki. Innan hvers ættbálks geta verið mismunandi ættir.
![Thumb](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Biological_classification_L_Pengo_Icelandic.svg/150px-Biological_classification_L_Pengo_Icelandic.svg.png)
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