Ultimogeniture, also known as postremogeniture or junior right, is the tradition of inheritance by the last-born of a privileged position in a parent's wealth or office. The tradition has been far rarer historically than primogeniture (sole inheritance by the first-born) or partible inheritance (division of the estate among the children).
Ultimogeniture might be considered appropriate in circumstances where the youngest child had been assigned the role of "keeping the hearth", taking care of the parents and continuing at home, whereas elder children had had time and opportunity to succeed in the world and provide for themselves. In a variation on the system, elder children might have received a share of land and moveable property at a younger age, for example when marrying and founding their own family. Ultimogeniture might also be considered appropriate for the estates of elderly rulers and property-owners, whose children were likely to be mature adults.
There are several disadvantages for realms and families that adopt ultimogeniture. One such drawback is the fact that elder siblings, especially the first born of the relevant gender, will be heavily incentivized to sidestep the tradition, even more so if primogeniture inheritance is a familiar concept. And since the elder siblings likely have more time and opportunities to gain power, wealth, experience, and influence prior to the inheritance—simply because they were born earlier—ultimogeniture traditions are more likely to be disregarded or even discarded.
Coercion, assassination, fratricide, or even patricide can be committed, backed by all the advantage an elder sibling would have, such that ultimogeniture is sidestepped. To clarify, those who stand to gain by ignoring the stipulation under ultimogeniture are more likely to have the facility to do so, when compared to other succession laws. For example, under primogeniture tradition, the younger siblings will stand to gain if they can bypass said tradition, but the elder siblings still have the aforementioned advantages, so primogeniture traditions tend to persist.