Sequential index numbers between 0.0 and 1.0 that allow arbitrarily inserting new numbers in between. They can possibly used for disk-based and other special containers, where adding a new element without changing the indexes of the other elements is important. Conceptually, SequentialIndex denotes a path to an element in an imaginary binary tree, with a '1' at the end. Except for 0.0 and 1.0, which are logically on the left or on the right of the entire tree. So logically, the tree looks roughly like this: 0.0 1.0 / /----------/ 0.1 / \ /--/ \--\ / \ 0.01 0.11 / \ / \ 0.001 0.011 ... Note that 0.0 is not connected to any other node, but it is still logically smaller than all nodes.