Porcelain dolls that have a porcelain, composite or paper-mache body and some plastic dolls are made in 6 pieces (head, body, 2 arms, 2 legs). There is a hook in the head and each of the 4 limbs and they are held to the body by strong elastic. Over the years the elastic stretches and loses its tension, or may even break, and the doll has a very "relaxed" look or may even fall to pieces. At this stage, the doll needs re-stringing.
This repair may not be straightforward because some of the hooks may have been lost or the top of the limbs, or the sockets they fit into, may have become damaged over the years.