Sister chromatids are joined by a centromere. Sister chromatids are two copies of each chromosome that are formed after DNA replication and they then enterthe process of mitosis. Centromere positions can vary--they can be metacentric which means that if the two chromosome arms are roughly the same size, it seems to be located in the middle. Submetacentric is when the arm's length is unequal and the centromere is off center holding the two chromatids together. Acrocentric is when the short arm is very tiny and difficult to observe and it seems like the centromere is connecting the two chromatids at the top or "peak".
The centromere joins chromatids (chromatid pairs)