Almost everyone who matters in the Mahabharata is descended from one king, Shantanu of Hastinapura, and the war is a quarrel between two branches of his house. The complication is that for three generations running, the heirs are not fathered by the men whose sons they are called.

Where does the line begin?

The dynasty is traced from the lunar line through Yayati and his son Puru. Some generations later comes Bharata, son of King Dushyanta and Shakuntala, from whom the family takes the name Bharata and after whom, the tradition holds, the land itself is called Bharatavarsha. Bharata passes over his own sons and adopts an heir, the first of several times this house refuses the obvious succession.

From Bharata's line comes Hastin, credited with founding Hastinapura, and then Kuru, who gives his name both to the clan and to the field of Kurukshetra where it destroys itself. Every warrior called a Kaurava in the wider sense - which includes the Pandavas - is a descendant of Kuru.

Who were Shantanu's wives and sons?

Shantanu married the river goddess Ganga, who bore him eight sons and drowned the first seven at birth. The eighth she kept and raised, and returned to Shantanu as Devavrata, the finest prince of his generation.

Later Shantanu wanted to marry Satyavati, a fisherman's daughter. Her father would agree only if her sons inherited. Devavrata renounced the throne, and then, because his own children would still be a threat, swore lifelong celibacy. The gods called him Bhishma, "the terrible", for the oath, and his father gave him the power to choose the hour of his death. That vow is the hinge of the whole epic: it empties the throne that everyone afterwards fights over.

Satyavati bore Shantanu two sons, Chitrangada, who was killed young by a gandharva, and Vichitravirya. Bhishma won three princesses of Kashi at their swayamvara for his half-brother; the eldest, Amba, was in love with another king and was released, and her pursuit of revenge against Bhishma ends with her rebirth as Shikhandi. Vichitravirya married the other two, Ambika and Ambalika, and died of consumption without children.

How were Dhritarashtra, Pandu and Vidura born?

By niyoga, the old custom by which a widow could bear a son to a kinsman on her dead husband's behalf. Bhishma would not break his oath, so Satyavati summoned Vyasa, the son she had borne to the sage Parashara before her marriage.

Ambika closed her eyes when the ascetic came to her, and her son Dhritarashtra was born blind. Ambalika turned pale, and her son Pandu was born pale and sickly. Asked for a third attempt, Ambika sent her maid instead, and that woman, undisturbed, bore Vidura - healthy, wise, and permanently ineligible for the throne because his mother was a servant. Vidura becomes the epic's most consistently correct voice and is never in a position to be obeyed.

So the succession crisis is baked in from the start. The eldest is blind and cannot rule, the second is cursed and dies early, and the third is the son of a maid.

Why are the Pandavas called sons of gods?

Pandu killed a sage who had taken the form of a deer while mating, and was cursed to die the moment he approached a woman in desire. His wife Kunti had received a mantra from the sage Durvasa as a girl that could summon any god to give her a son, and had already tested it on Surya before her marriage.

At Pandu's asking she used it three times: Dharma gave her Yudhishthira, Vayu gave her Bhima, Indra gave her Arjuna. She taught the mantra to Pandu's second wife Madri, who summoned the twin Ashvins and bore Nakula and Sahadeva. Pandu died when the curse caught him, and Madri burned on his pyre. The five brothers are therefore sons of Pandu in law and sons of gods in fact, and they inherit those fathers' gifts: Yudhishthira's truthfulness, Bhima's strength, Arjuna's skill, the twins' beauty and knowledge.

Who are the hundred Kauravas?

Gandhari, princess of Gandhara, married Dhritarashtra and bound her own eyes with a cloth when she learned her husband was blind, keeping it on for the rest of her life. Her pregnancy lasted two years and produced a mass of flesh that Vyasa divided into a hundred and one parts, incubated in jars; from them came Duryodhana, ninety-eight further sons, and a daughter, Dussala, who married Jayadratha.

Duryodhana and his brother Dushasana drive the action; Vikarna is the one Kaurava who objects aloud when Draupadi is dragged into the hall. Dhritarashtra also had a son by a vaishya woman, Yuyutsu, who crossed the field to the Pandavas before the first day and was one of the few Kauravas to survive the war.

Gandhari's brother Shakuni came to Hastinapura with her and stayed, and it is his dice and his counsel that turn Duryodhana's resentment into a war.

Where does Karna fit?

Karna is the eldest Pandava, and nobody knows it until he is dead. Kunti bore him to Surya before her marriage, set him adrift on a river in a basket out of fear, and he was raised by Adhiratha, a charioteer of Hastinapura, and his wife Radha. He therefore grows up a suta, of mixed caste by the epic's reckoning, and spends his life being refused contests he is qualified for.

Duryodhana makes him king of Anga in one gesture on the day of the princes' weapons trial, and Karna's loyalty to him never wavers afterwards - not when Krishna offers him the empire if he changes sides, and not when Kunti herself comes to the riverbank to tell him who he is. He promises her he will spare four of his five brothers, so that she will have five sons whatever happens.

Karna's presence turns the war into a fratricide twice over: the cousins are fighting each other, and the man leading the Kaurava army in its last days is the brother of the men he is trying to kill.

Krishna is their first cousin. His father Vasudeva of the Yadava clan is Kunti's brother, which makes Krishna and Balarama the Pandavas' maternal cousins, and Krishna's sister Subhadra becomes Arjuna's wife. Krishna is also related to the other side: his cousin Kritavarma fights for Duryodhana, and Krishna gives his whole Narayani army to the Kauravas while going himself, unarmed, to the Pandavas.

The Pandavas' other marriages widen the family further. All five are married to Draupadi, daughter of Drupada of Panchala, and each has a son by her. Bhima fathers Ghatotkacha on the rakshasi Hidimbi; Arjuna fathers Iravan on the naga princess Ulupi and Babruvahana on Chitrangada of Manipura.

Who survives to carry the line?

One unborn child. Arjuna's son Abhimanyu, born to Subhadra, is killed on the thirteenth day of the war at sixteen. His wife Uttaraa, daughter of the king of Virata, is pregnant, and Ashwatthama's last weapon is aimed at that womb. Krishna revives the child, who is named Parikshit, "the tested one".

Parikshit inherits the kingdom when the Pandavas leave for the mountains, and dies of a serpent bite after a curse. His son Janamejaya holds the great snake sacrifice to avenge him, and it is at that sacrifice that the Mahabharata is recited for the first time to an audience. The family tree ends by producing the listener the poem is told to.

Key facts

Common ancestor: Kuru, descendant of Bharata
Last king before the split: Shantanu
Biological grandfather of both branches: Vyasa, by niyoga
Pandavas: Yudhishthira, Bhima, Arjuna, Nakula, Sahadeva
Kauravas: 100 sons of Dhritarashtra and Gandhari, plus Yuyutsu
Hidden eldest: Karna, Kunti's son by Surya
Heir after the war: Parikshit, son of Abhimanyu and Uttaraa

Sources

Mahabharata, Adi Parva, in the translation of K. M. Ganguli (1883-1896).
Bibek Debroy, The Mahabharata, vol. 1 (Penguin, 2010).
Irawati Karve, Yuganta: The End of an Epoch (1969).

Where the guides summarise a disputed point, they say so. How this site is put together.