Each doll in this sequence represents one of the M_i. While this is perhaps a fairly straightforward visual metaphor for an artinian module, it is important to note that while this photograph implies that the M_i are decreasing in "size" (cardinality), this need not be the case: for instance, all of the submodules could be countably infinite. Rather, we have that each M_i contains M_i+1. While these dolls obviously do not contain one another in the photograph, the use of matryoshka dolls gives a nod to the notion of containment. As we move from left to right, the dolls eventually become the same in size, reflecting the fact that a descending sequence of submodules of an artinian module must eventually become stationary: that is, after some point in the sequence, the M_i are all identical.