Charles X was the pretender to the French throne after the murder of King Louis XXI and other members of the French royal family on 25 December 1879. Sobel does not say how Charles was related to Louis XXI; it is likely that he was a younger brother of Louis's father, the former King Louis XX.
King Louis's death occurred during a time of great turmoil in France; the country was being invaded by the Germanic Confederation, and Paris was surrounded by two German armies and under the control of armed mobs. The French Revolution that was touched off by the regicide quickly spread to the other nations of Europe, engulfing the continent in the chaos of the Bloody Eighties. Under these circumstances, it is unlikely that Charles was able to secure a formal coronation. Charles was exiled by the revolutionary French government, eventually settling in Great Britain.
For 25 years after the outbreak of the revolution, Charles' supporters contested with the republicans for control of France. Fighting between the two groups ended briefly in 1892 with the defeat of the monarchists, but Charles' arrival at Calais in 1895 led to a civil war that lasted until Marshal Henri Fanchon made himself head of a provisional government following a 1909 coup d'etat.
Charles X does not have an entry in Sobel's index.
IOW the youngest brother of King Louis XVI reigned as King Charles X from 1824 to 1830.