Why Queen Victoria's proclamation left room for an unborn rival
When Victoria inherited the British throne from her uncle William IV, her proclamation carried an odd caveat protecting any child his widow might still bear. Queen Adelaide was 44, and a posthumous baby of either sex would have outranked Victoria. That gap is the difference between an heir presumptive and a true heir apparent.
An heir apparent stands first in line and cannot be pushed aside by anyone's birth, provided the succession laws stay the same. An heir presumptive is also first for now but could be displaced if a closer heir arrives. Legal systems generally assume a child can always be born, whatever a person's age or health, so someone who is effectively certain to inherit may still, on paper, be only presumptive.
Under male-preference rules, daughters rank behind brothers of any age, so even an only daughter normally stays presumptive. Elizabeth II was in that position under her father George VI, since a son would have leapfrogged her. A daughter can still become heir apparent if her father, the heir, dies leaving no sons, once it is certain his widow is not pregnant. In one special case Anne became heir apparent while William III reigned alone after Mary II's death, because the 1689 settlement let only Mary's line carry on the succession.
Gender-blind succession is recent. Sweden adopted it first in 1980, which briefly made the infant Carl Philip heir apparent and then transferred that status to his older sister Victoria, a rare loss of the position without anyone dying. The Commonwealth realms followed in 2015 under the 2011 Perth Agreement, and heirs such as Princess Catharina-Amalia of the Netherlands and Princess Elisabeth of Belgium now stand first as eldest children.
Heirs apparent usually do inherit, but deposition can intervene. Parliament passed over the infant son of James II in favour of his Protestant half-sister Mary, and Sweden's Crown Prince Gustav lost his place in 1809 when his father was overthrown.
Source: Heir apparent
