I wrote a post on this topic all the way back in June 2007 (yowza, that's close to a decade and a half ago now!), and it's had an absolute ton of page views. I've decided it's time to write another one.
In the thirteenth and fourteenth centuries, the children of kings, but nobody else, had the right to be called Lord and Lady from birth. Before his accession to the throne, Edward II was always known as Lord Edward, and after February 1301 was called 'Lord Edward, Prince of Wales' (Dominus Edwardus Princeps Walliae in Latin or (Mon)sire Edward prince de Galles in French), or often just 'the Prince' for short. He was never, however, called 'Prince Edward', an important distinction. Edward's wife Isabella, daughter of Philip IV, king of France and Joan I, queen of Navarre, was 'Lady Isabella' or more correctly 'my lady Isabella' (ma dame Yzabel) before she married Edward, and as his wife, was called 'Lady Isabella, queen of England'. A letter to Edward II from the bishop of Norwich and the earl of Richmond in the spring of 1325 talked of 'the queen of England, your consort, our lady' (la royne Dengleterre, vostre compaigne, nostre dame). It was also common to refer to Edward as 'our liege lord' and to Isabella as 'our liege lady'.
Edward's accounts from the year 1293/94, when he was nine years old, happen to survive. In them, he always appears as Lord Edward, while his Lancaster cousins, who stayed with him for a while, were referred to as 'Thomas and Henry the sons of Lord Edmund', i.e. Henry III's younger son Edmund, earl of Lancaster and Leicester. The Lancaster boys were grandsons of a king but sons of an earl, and so had no right to the title which their father had held from birth. The accounts of the joint household of Edward II's brother Henry (1268-74) and sister Eleanor (1269-98), and their cousin John of Brittany (1266-1334), son of Edward I's sister Beatrice, also survive for a few months when they were small children. Henry was always called Dominus Henricus, Eleanor was Domina Alianora, but John, grandson of King Henry III and son of Duke John II of Brittany, was simply Johannis or, rather amusingly, Britonis, meaning 'the Breton'.
The same applies to Edward's brother-in-law Jan of Brabant (1275-1312), who married Edward's sister Margaret in 1290 and was the son and heir of Duke Jan I of Brabant. A roll of expenses of Jan's household survives from an unknown date probably in the early 1290s, and he is simply called Jehan or Jehans in it. He hadn't yet been knighted, and was only the son-in-law of a king, not the son. By contrast, an indenture regarding the household of Edward I and Marguerite of France's first child Thomas of Brotherton, dated 7 January 1301, refers to the 'wardrobe of Lord Thomas the king's son', garderobe domini Thome filii regis. Thomas was then only seven months and six days old, but because he was the son of a king, the document calls him Lord Thomas. I assume it was the sixteenth century when the children of kings began to be known as Prince and Princess, and they certainly weren't in the fourteenth. Contrary to popular modern belief, Isabella of France was not a princess. That title was not yet given to the daughters of kings, and Isabella married Edward II after his accession to the throne and was never Princess of Wales.
It's interesting to note that Edward II has always been very closely associated with his birthplace in North Wales. In January 1330 a little over two years after his funeral, his friend William Melton, archbishop of York, referred to him as 'our liege lord Edward of Caernarfon' (n're seign' liege Edward de Karnarvan) in a letter to the mayor of London. Some of Edward's sisters are also known by their places of birth, Joan of Acre in particular, and also Elizabeth of Rhuddlan. Edward's half-brothers and his children were always known by their places of birth in their own lifetimes - e.g. Edmund of Woodstock, Eleanor of Woodstock, Joan of the Tower - as were his grandchildren, the twelve children of Edward III and Queen Philippa, e.g. Lionel of Antwerp, Margaret of Windsor, Edmund of Langley. With the exception of Richard of Bordeaux, however, and sometimes Henry of Bolingbroke, this custom wasn't carried on to Edward III's grandchildren, who instead were mostly known by their fathers' ducal titles: Philippa, Elizabeth and Katherine/Catalina of Lancaster, Philippa of Clarence, Humphrey and Anne of Gloucester, Constance of York, and so on. Elizabeth of Lancaster, countess of Huntingdon (1363-1425), the second daughter of John of Gaunt and Blanche of Lancaster, almost always called herself 'Elizabeth Lancastre' without a de or 'of', as though it was her family name. I find it interesting that she continued to refer to herself this way throughout her long and apparently very happy third marriage to Sir John Cornwall, and did not use his surname. When attending their grandmother Queen Philippa's funeral in early 1370, Elizabeth and her older sister Philippa (b. 1360, later queen of Portugal) were called 'the two daughters of Lancaster'.
The future Edward III (b. 1312) was created earl of Chester at a few days old but was never made prince of Wales - the next time the title was used was for Edward III's eldest son Edward of Woodstock in 1343 - and it is incorrect to call him 'Prince Edward' or 'Edward the prince'. He and his brother John of Eltham (b. 1316) were called Monsire Edward de Wyndesore ('Lord Edward of Windsor', his birthplace) and Monsire Johan Deltham, or d'Eltham as it would be in modern French, during their father's reign. A letter to Edward II of 1325 refers to his and Isabella's daughters Eleanor of Woodstock (b. 1318) and Joan of the Tower (b. 1321) as mes dames vos deus files, 'my ladies, your two daughters', and in Edward's accounts John of Eltham appears as Monsire Johan Deltham fuiz le Roi, 'son of the king'. Isabella appears in her husband's accounts as ma dame la roigne or 'my lady the queen', and in royal documents from the reigns of Edward I, Edward II and Edward III, the words ma dame used alone always refer to the queen.
Edward II was only very rarely called 'Edward II' in his own lifetime, and was usually 'King Edward son of King Edward'. His father was 'King Edward son of King Henry [III]'. On Edward III's accession in 1327, it appears that clerks decided that they couldn't keep writing 'King Edward son of King Edward son of King Edward' all the time forever and ever, sometimes adding 'son of King Henry' for good measure, because their hands were going to cramp or whatever. Therefore, they decided to start calling the young king 'Edward the third' (the third English king of that name after the Norman Conquest of 1066).
'Your Majesty' was a later title, as indeed were 'Your Highness' and 'Your Grace'. Edward II was addressed fairly simply in speech as 'Sire' or 'my lord king', or even just 'my lord' or 'my king'. In writing, it was polite to refer to him as 'our lord the king' or 'the lord king', and you can tell that the author of the Vita Edwardi Secundi was becoming utterly exasperated with Edward in and after 1322 when he started referring to him merely as 'the king' (rex in Latin) rather than 'the lord king' (dominus rex) as he had always done before. An extant letter sent by Hugh Despenser the Younger in 1319 rather discourteously refers to Edward simply as 'the king' (le Roy in the French original) and not as 'our lord the king' (nostre seignur le Roy) as Hugh always did in his later letters.
When talking to other people, even pretty high-ranking ones, Edward II appears to have addressed them by their first names. The Vita Edwardi Secundi records a conversation he had with Sir Andrew Harclay, sheriff of Cumberland, in early 1322, and Edward just called the sheriff 'Andrew' not 'Sir Andrew'. How he addressed earls when talking to them, or how he referred to them when they weren't present, isn't something I've been able to find evidence for. A few decades later, Edward's great-grandson Richard II addressed Thomas Beauchamp, earl of Warwick, as 'Thomas of Warwick'. I should probably note at this point that there were no dukes in England until 1337, except that either the king or his heir was duke of Aquitaine in south-west France. The first English duke was Edward III's eldest son Edward of Woodstock, made duke of Cornwall in 1337 and prince of Wales in 1343. The second was the royal cousin Henry of Grosmont, made duke of Lancaster in 1351, and until 1397 when Richard II went slightly mad creating lots of new duketti ('little dukes', as contemporaries contemptuously called them), the only other English dukes were Edward III's younger sons Lionel, John, Edmund and Thomas. The first English duchess (excepting that the queen of England was duchess of Aquitaine) was Henry of Grosmont's oddly obscure wife Isabella Beaumont (d. 1359/60) in 1351, because Edward of Woodstock, the first English duke, didn't marry until 1361. The first English duchess in her own right was Edward II's niece Margaret of Norfolk (b. c. 1322) in 1397.
In writing, people tended to be rather more polite and formal than when speaking, which is usually the case, of course. I somehow find it quite funny that in his extant letters of 1323 to 1325, Hugh Despenser the Younger always punctiliously referred to Roger Mortimer of Wigmore by his correct title, 'Sire Roger de Mortimer', even when telling people that Roger was planning an invasion of England and when Hugh certainly knew that Roger wanted him dead.
Hugh himself was addressed in letters of the 1320s, when he was the king's mighty favourite, in the most fawningly obsequious terms: 'the very noble man, my very honourable lord'; 'the very noble and wise man, his very dear and very honourable lord'; 'my very honoured and very dread lord'; 'to the noble man, very honourable lord, all manner of reverences', and so on. There's only one extant letter I know of sent from Hugh Despenser to Edward II, which dates to c. 1324 - and is more of a note than a proper letter - and it opens 'Honours and reverences, very honourable lord'. Others tended to address Edward in their correspondence as 'Very dear, very dread lord' (trescher tresredotable seignur), and throughout their letters often addressed him in the third person, as in 'Very dear, very dread lord, your humble subject recommends himself to your very high lordship. May it please your very high lordship to know...'. Three of Edward's clerks outdid themselves in a letter of July 1324, which opened 'To the very noble and very honourable lord, our lord, Lord Edward, by the grace of God king of England...'.
Edward II's own letters mostly began 'Edward, by the grace of God king of England, lord of Ireland and duke of Aquitaine, to our dear and faithful [name and title], greetings', sometimes 'greetings and dear affection'. It was conventional in the fourteenth century to end a letter with 'May God keep you' (A Dieu qe vous gard in French), or 'May the Holy Spirit have you in his keeping', or, rather more hair-raisingly but surprisingly common, 'May God grant you vengeance over your enemies'.
Below, part of a letter from the Gascon lord Arnaud Caillau to Hugh Despenser the Younger in 1325, with two closing salutations: 'May our Lord increase your honour, and grant you a good and long life, and give you vengeance over your enemies wherever they may be', and a few lines later, 'May our Lord have you in his keeping and guard you from all evils'.