It was quite common among the elite social classes. William the Conqueror was known in Normandy as William the Bastard because his mother was the daughter of a tanner and his father was the Duke of Normandy. His status as a bastard did not inhibit him from becoming his fathers heir to the duchy of Normandy and eventually the King of England. However, after his ascension to the throne laws were passed to inhibit a bastard from becoming King. Royal bastards could receive royal favors and titles. Henry VIII bastard son, Henry FitzRoy, became the Duke of Richmond and Somerset and the Earl of Nottingham. Side not the name FitzRoy means "son of the king" and was a common surname given to royal bastards. There are cases where the bastard line became legitimate after the mother and father officially married. Some examples are the Beaufort and Tudor lines that were legitimized upon royal approval and the marriage of the mother and father. I'm sorry I can only speak on royal English illegitimate lines. I believe France had even looser rules and practices concerning royal bastards. The French did have the title of maitresse-en-titre, an official title held by the mistress of the French King.

In Family and Household in Medieval England, Peter Fleming notes that illegitimate children were 'not unusual in landed society and were often provided for', but they were more likely to be acknowledged if there were no surviving legitimate children. In theory, illegitimate children were barred from inheriting their parents' property, though sometimes would do if no legitimate children survived.

Among the middle and lower classes, the rate of illegitimacy was low; as Fleming not-so-helpfully puts it: 'Exact rates of illegitimacy are impossible to determine before the era of parish records in registration, but they are likely to have been relatively low, certainly be modern standards.' Marriage was an easy process, and sexual relations were understood to be sanctioned by marriage if not betrothal; I would imagine that those couples who became pregnant by premarital sex would simply have gotten married and solved the problem.

Bastardry was only a social inhibitor in societies which only valued children born in wedlock. For example, the Welsh didn't seem to care about bastardry despite being fairly orthodox in their beliefs. In the Welsh law books originally written (probably) in the late twelfth- and thirteen-centuries (only extant in later MS) the jurists claim that the son should not be punished form his fathers errors. Dafydd ap Owain of Gwynedd married Henry II's illegitimate half-sister (Emma), and later John I would marry his bastard daughter (Joan) to Llywelyn ap Iorwerth of Gwynedd. These were coups for the Welsh rulers, and the latter case proves another point about bastardry in the Middle Ages. After Llywelyn wrote to Pope Honorious III claiming he was getting rid of this vile (as it was reviled in much of Christendom) of allowing bastards to inherit - but could the Pope please legitimise his wife Joan? Well Llywelyn might have had a genuine pious desire to get rid of this Welsh custom he might have had an ulterior motive - I discuss it in point two - but the fact is that the custom persisted until Wales was annexed c.62 years later in 1283/4 and came under English rule.

One illustrative example would be John of Gaunt son of Edward III, uncle of Richard OO. His legitimate son was later Henry IV.

However he sired 4 children by his mistress Katherine Swynford. He later married her and her children, surname Beaufort were legitimized but barred from succession. Nonetheless they became the Lancaster faction one of whose members (thru his mother) eventually married a York and ascended to the English throne as Henry VII. It sparked a faor amount of contention known as The Cousins War.

Elizabethan but set a few years earlier...shakespeare's infamous bastards monologue.

Thou, nature, art my goddess; to thy law My services are bound. Wherefore should I Stand in the plague of custom, and permit The curiosity of nations to deprive me, For that I am some twelve or fourteen moon-shines Lag of a brother? Why bastard? wherefore base? When my dimensions are as well compact, My mind as generous, and my shape as true, As honest madam's issue? Why brand they us With base? with baseness? bastardy? base, base? Who, in the lusty stealth of nature, take More composition and fierce quality Than doth, within a dull, stale, tired bed, Go to the creating a whole tribe of fops, Got 'tween asleep and wake? Well, then, Legitimate Edgar, I must have your land: Our father's love is to the bastard Edmund As to the legitimate: fine word,--legitimate! Well, my legitimate, if this letter speed, And my invention thrive, Edmund the base Shall top the legitimate. I grow; I prosper: Now, gods, stand up for bastards!