Customs a reader needs

The story never stops to explain its rules. Where only a scene shows one, the scene is named.

Geasa and binding words

The word geasa appears only in two of Kinsella's notes, both on "Exile of the Sons of Uisliu". Derdriu seizes Noisiu by the two ears and says they will be ears of shame and mockery if he does not take her with him. The text reads "binding him," and a note says those words are not in the original: her words put Noisiu under bond, or geasa, to do what she asked. A second note says Kinsella added a sentence and a few phrases to bring out the geasa governing the action, because the original is so spare as to be obscure. The sons of Uisliu have sworn to eat no food in Ireland until they eat Conchobor's food first. Fergus, who was to come as their pledge of safety, is held back by an old oath that stops him refusing a number of ale feasts. The sons go on without him and are killed on the green at Emain. The notes do not say what happens to someone who breaks a geis,.

Fosterage

Kinsella's note says families of consequence normally put their children out to foster-parents, who were not necessarily of inferior status. When Cúchulainn is born, Sencha, Blai Briuga, Fergus and Amargin each claim the right to rear him in a speech. The judge Morann gives each a part, and the boy goes to Amargin and Finnchaem.

Fosterage still binds when the war comes. Fergus, pressed to fight his foster-son, says he would not use a sword on him, and Cúchulainn retreats on condition that Fergus gives way to him at the last battle. Ferbaeth calls Cúchulainn his foster-brother, "sworn to me for ever," and goes out against him anyway because he promised Medb. Ferdia, who trained beside Cúchulainn under Scáthach, is his foster-brother at the ford.

Sureties and hostages

Before he agrees to fight Cúchulainn, Ferdia asks Medb for a surety. She tells him to pick any hostage he likes, and he takes six princes. He tells his followers the terms: he fights Cúchulainn next day or fights the six, and the six are sent to make sure Medb keeps her promises if he wins. A Munster king says Finnabair was promised to him with fifteen hostages as a guarantee.

Single combat at a ford

Kinsella's note says defending a boundary at a river crossing is natural, but warriors also seem to choose a ford as an arena for certain combats. He names the combats with Fraech, Lóch and Ferdia, and reports a suggestion that a ford combat has something of a divination rite in it.

In "Single Combat" Cúchulainn refuses each of Ailill's offers. Fergus works out what he wants instead: one fight at a time in the ford, and no cattle taken from the ford for a day and a night after each. Ailill agrees, since it is easier to lose one man a day than a hundred a night. When Cúr mac Daláth falls, Fergus reminds the camp that they are bound by the pact.

Fair fight is part of the rule. Six sons of kings go at Cúchulainn together and die. Laeg tells him to bring his sword to Medb's meeting, since a warrior without weapons is treated under the rule for cowards, and fourteen men attack him there. Fergus says they must stop breaking the rule of fair fight. Against Ferdia the choice of weapons alternates by day: "I chose yesterday."

Chariots, hoops and ogam

Medb's charioteer turns the chariot to the right, with the sun, to draw down the power of the sign. Kinsella's note says turning right attracts good fortune and turning the left side of the chariot is an act of insult or challenge. Etarcomol turns the left chariot-board against Cúchulainn, who says he cannot refuse that.

A spancel-hoop, Kinsella says, is a twig ring for spancelling horses and cattle, left at fords or boundaries as a message of challenge. Ogam is an incised lettering on stone or wood. Cúchulainn leaves a hoop with an ogam message on a standing stone: the army goes no further unless a man can make a hoop like it with one hand.

Satire

Kinsella's note says Leborcham, as a satirist, would have more than usual freedom, since her verses might bring harm. She "couldn't be kept out" of Derdriu's enclosure. Ailill's satirist Redg demands Cúchulainn's javelin and threatens to take away his good name. Cúchulainn throws it through his head. Medb sends satirists to Ferdia to bring the blushes to his cheek, and he comes in dread of being put to shame.

Hospitality and wealth

Kinsella's note says a briugu, or hostel keeper, had charge of a great house of hospitality, owned large herds and many ploughs, and enjoyed almost the same dignity as a noble. Blai Briuga boasts he could feed all the men of Ireland for a week or ten days. Dáire mac Fiachna feeds Medb's messengers until they are drunk and noisy, then tells them only his habit of not murdering messengers or travellers keeps them alive.

Wealth is counted in animals and bondmaids. In "The Pillow Talk" Ailill and Medb match sheep, horses, pigs and cattle to settle who owns more, and a chariot is worth thrice seven bondmaids. Medb offers fifty yearling heifers for the loan of the Donn Cuailnge. Emer's dowry is paid the day after her wedding, and Cúchulainn receives his honour-price.