The Crown Prince of Saudi Arabia is the second most important position in Saudi Arabia, second to the king, and is his designated successor.
Currently, the Crown Prince assumes power with the approval of the Allegiance Commission after he is appointed by the king. This system was introduced to the country in the Abdullah Era. In the absence of the king, an order is issued to have the prince manage the affairs of the state until the king's return. The order changes the prince's title temporarily from the Crown Prince, Deputy Prime Minister to Vice Custodian of the Two Holy Mosques until the return of the king to the kingdom.
The last Crown prince of the Second Saudi State was Abdulaziz ibn Abdul Ramhan ibn Saud, who lost the title when his father lost his state to the House of Rashid in the 19th century. His father abdicated the pretended throne to him in 1900, and began his famous wars of conquest of what would become the third Saudi state. When had taken enough land to become recognized as a genuine Emir, he designated his eldest son Turki as his heir. When Turki died during the flu pandemic of 1919, his only child was still in utero, and Abdulaziz designated his second son Saud to be heir and that further succession would be brother to brother.
When the Unification of Saudi Arabia was complete in 1932, Abdulaziz had himself declared king, and a year later Saud was declared Crown Prince.
When the old king died in 1953, Saud became king and his brother Faisal, as planned was declared Crown Prince. Then things became somewhat complicated. The King and his heir began feuding over whether or not King Saud was competent or not. Feisal took the title "Prime Minister" for a while, and on occasion took over the government against the king's wishes. Saud fought back, and things came to a head in 1964, when Feisal deposed his brother and became king.
The next in Line, Prince Mohammed, was deemed unsuitable, and was de jure Crown Prince for a short time before he was deposed in favor of Prince Khalid in 1965.
With Khalid not all that interested in being king, his crown prince, Fahd, took the reins of power as de facto ruler of the country. When King Fahd had a stroke in 1995, Crown Prince Abdullah became the formal Regent for the balance of the reign.
As the nation became a gerontocracy in the 2000s and early '10s, three Crown Princes died of old age in rapid succession. In the meantime, more and more princes were passed over. In January 2015, Ibn Saud's last son, Muqrin, became Crown Prince, only to be ousted three months later in favor of his nephew, Mohammed bin Nayef.
The honorific title of "Deputy Crown Prince" only dates from 2014, but the position, under the title of "Second Deputy Prime Minister" goes back to 1965, in order to designate who was the senior prince not excluded from the throne.
By late 1964, it became obvious to those at court that Crown Prince Mohammed was not suitable for the throne and that King Faisal wanted to also bypass the next three in line and install Prince Fahd to be his heir. However, a faction loyal to the soon-to-be-out Crown Prince wouldn't have it, so as a compromise, Prince Khalid, who was considered a nonentity, would be next in line; princes Saad and Nasir, who had no qualifications aside from seniority, would be skipped as planned, and that Fahd would take the title of "Second Deputy Prime Minister" to cement his place as second in line.
When King Faisal was murdered in 1975, King Khalid designated Prince Abdullah, who was next in line anyway, as second deputy.
As King Khalid became ill with old age, the question of who would succeed Abdullah as Second Deputy became more pressing. Prince Musa'id, whose son had murdered King Faisal, long knew he was out and Prince Bandar demanded and got, a huge bribe to stand down. Prince Sultan, despite opposition, was appointed when the old king died.
Once a Prince's place was cemented in the line of succession, he was impossible to remove. Prince Sultan attempted a coup against Crown Prince Abdullah, who was regent for the ailing King Fahd, in 1995, and he remained in place until he became Crown Prince in 2005. Competition for the position became intense when Fahd, then Sultan died a few years later, then Crown Prince Nayif less than a year after that. The youngest brother of all, Muqrin, became second deputy in 2013, and received the honorific "deputy crown prince" a year later.
With the internal politics of the House of Saud being "clear as mud" the seniority system came to an end when scores of grandsons were bypassed in favor of Muhammad bin Nayef.