Stewards of Gondor
Steward (Arandur in Tolkien's Quenya language) was the traditional title of a chief counsellor to the Kings of Gondor. The office arose early in the Third Age during the reign of King Rómendacil I. After the Stewardship of Húrin of Emyn Arnen the office was awarded only to his descendants (the House of Húrin). After Mardil, in the absence of the Kings, the office by custom became hereditary, passing from father to son or nearest male relative, like the Kingship.
The House of Húrin was founded by one Húrin of Emyn Arnen, Steward to Minardil, the twenty-fifth King of Gondor (reigned T.A. 1621 to 1634). They were of high Númenórean blood, but not descendants of Elendil in the ruling line.
Although not considered a Ruling Steward, Pelendur was the first to effectively rule the kingdom, doing so for one year after the death of King Ondoher and his sons while fighting the Wainriders (T.A. 1944). He played a key role in influencing the Council of Gondor to choose Eärnil over Arvedui of Arthedain, thus maintaining the line of the heirs of Anárion.
Eärnil's son Eärnur rode against the Witch-king leaving no heir. Eärnur did not return, and since his fate was not known, the Steward Mardil Voronwë administered Gondor in the name of the absent King, thus becoming the first of the Ruling Stewards. Mardil's descendants administered Gondor as Ruling Stewards for 25 generations. The stewards from Eradan to Dior ruled during the period known as the Watchful Peace. The Ruling Stewards ruled in place of the King, swearing an oath to do so "until he shall return". In practice they exercised all the powers formerly held by the Kings, but avoided associating themselves with any of the symbols of kingship. They sat on a simple chair of black stone placed on the lowest step of the dais surrounding the throne, wore no crown, and held no sceptre; a white rod topped with a gold knob served as the emblem of their office. In place of the royal flag they flew the banner of the Stewards, which was white with no charge.
The Stewards watched over the throne until it could be reclaimed by a true King of Gondor, an heir of Elendil. When asked by his son Boromir how much time must pass before a Steward could become a King, if the King did not return, Denethor II replied: "Few years, maybe, in other places of less royalty ... In Gondor ten thousand years would not suffice."  After generations of rule by the Stewards, they hardened their hearts against the return of an heir of Elendil.
Before the Line of Kings failed two conditions applied to the Steward: He was not allowed to leave the realm or go to war, in effect ensuring smooth government while the King was away on campaign.
The family tree of the Stewards of Gondor can be found here.
Ruling Stewards of Gondor
The following is a list of the Ruling Stewards of Gondor. (All dates are from the Third Age, and are taken from Appendix A of The Lord of the Rings or (for the birth years) from The Peoples of Middle Earth, p 204–207.)
- Mardil Voronwë "the Steadfast", born 1960, Steward from 2029, ruled 2050–2080
- Eradan, b. 1999 (2080–2116)
- Herion, b. 2037 (2116–2148)
- Belegorn, b. 2074 (2148–2204)
- Húrin I, b. 2124 (2204–2244)
- Túrin I, b. 2165 (2244–2278)
- Hador, b. 2245 (2278–2395)
- Barahir, b. 2290 (2395–2412)
- Dior, b. 2328 (2412–2435)
- Denethor I, b. 2375 (2435–2477) – son of Dior's sister.
- Boromir, b. 2410 (2477–2489)
- Cirion, b. 2449 (2489–2567)
- Hallas, b. 2480 (2567–2605)
- Húrin II, b. 2515 (2605–2628)
- Belecthor I, b. 2545 (2628–2655)
- Orodreth, b. 2576 (2655–2685)
- Ecthelion I, b. 2600 (2685–2698)
- Egalmoth, b. 2626 (2698–2743) – grandson of Orodreth's sister, Morwen.
- Beren, b. 2655 (2743–2763)
- Beregond, b. 2700 (2763–2811)
- Belecthor II, b. 2752 (2811–2872)
- Thorondir, b. 2782 (2872–2882)
- Túrin II, b. 2815 (2882–2914)
- Turgon, b. 2855 (2914–2953)
- Ecthelion II, b. 2886 (2953–2984)
- Denethor II, b. 2930 (2984–3019)
At the death of Denethor II, Aragorn Elessar declared himself openly as the Heir of Isildur, and Denethor is considered the last of the Ruling Stewards. The army anticipated Aragorn's coronation by fighting in the name of the King Elessar, but by common consent the leader of the joint assault on Mordor was Gandalf. Until the army's return from Mordor, the Steward Faramir, Denethor's son, retained nominal control of Minas Tirith, though during his incapacity the city was actually commanded by Prince Imrahil of Dol Amroth and (when Imrahil departed with the army) by Húrin the Tall, Warden of the Keys. At Aragorn's coronation, Faramir surrendered his rod of office to the King, but the King returned it to him, and Elessar confirmed in Faramir and his descendants the office of Steward of Gondor, creating him in addition Prince of Ithilien.
Faramir was succeeded by his son Elboron as Steward of Gondor and second Prince of Ithilien. Faramir's grandson Barahir is mentioned in the Prologue of The Lord of the Rings as the author of The Tale of Aragorn and Arwen, but Tolkien does not indicate whether he was a Steward, or even the son of Elboron.
- Reported by Faramir in The Two Towers, "The Window on the West"
- The Lord of the Rings, Appendix A (iv), "The Stewards".
- Unfinished Tales, p 405
- The Lord of the Rings, Appendix A (ii).
- The Return of the King, "The Houses of Healing".
- The Return of the King, "The Steward and the King".
- The Peoples of Middle-earth, Vol. XII of The History of Middle-earth, p. 221.