This is both rather difficult and quite easy to answer. Any history of one of the major royal houses of Europe can point out what various younger sons did. For House Capet and Valois, I suggest Georges Duby's book, 'France in the Middles Ages'.

In most reasonably powerful houses, the younger sons would be given a title and land, and their power varied a huge amount. Jean II of France's younger brother, the Duke of Orleans, was a major power in 14th Century France, and indirectly lost the Battle of Poitiers for France by leading his troops away. In a similar vein, the third and fourth sons of Edward III of England founded the Houses of Lancaster and York, respectively, named for the duchies they were granted.

In summary, lesser sons tended to be parcelled off with land and titles in order to give them an active interest in the wellbeing of the realm, sate any desires for power and recognition, and also improve their administrative ability should the heir die.

For lesser lords, it's a touch more difficult. I've never been able to find a good source on this topic as a general concept, but from more widely-set books, such as Michael Powicke's 'Military Obligation in Medieval England' and Jonathan Sumption's 'The Hundred Years War', it is possible to piece together what was generally the done thing. Of course, it must also be included the poetic history of William Marshal, whom I love referencing, which is closer to your time period and a very good example.

The short answer is: just about everything. Some did, indeed, stick around their family estates and act as stewards, retainers, administrators and seneschals; but this wasn't seen as dishonourable, more like a man performing his duty. Others, those less martially inclined, would join the priesthood and often find the opportunity there for advancement through merit. But it was very, very common for younger sons to go off and fight, yes. As tourney patrons, as foreign mercenaries (Roger de Flor being a good example), as household knights for various lords, as crusaders, anything to gain some measure of prestige.

I hope this goes some way to answering your question.

Although your question was specific, and has been semi-answered, I would like to point out that there are forms of inheritance other than Primogeniture used throughout history, particularly European: Gavelkind, Ultimogeniture, Tanistry (elected heirs), Rota, etc. For example, a form of partible inheritence is what caused the Carolingian dynasty to split and ultimately form the lines for modern day France and Germany. In addition, a King's demesne would typically consist of multiple titles/land holdings. If he felt the need, he could grant one of those to a younger child (especially if it wasn't necessarily subordinate to his immediate realm) so the dynasty could carry on in the form of a dukedom, less prestigious kingdom, etc. Children were also important for forming political relationships or binding ties between dynasties/kingdoms and could be married off.

Barring all that, the other answers outline some of the more common outcomes, especially for fourth+, in line.

I have a knock off question that seems related enough to younger sons and inheritance to go here. I was reading Game of Thrones and in it a character (Samwell) is the eldest son but isn't good at fighting, etc. and his father doesn't see him as a fit heir. The younger son takes the fathers favor and Samwell is forced to go enter the Night Watch so that the younger son may inherit all titles and properties.

Did this sort of thing happen in real life? Are there any examples of older children not being seen fit and being persuaded/forced to do something like enter the church so that the titles and lands would pass to a younger child? Examples of younger children killing older siblings in order to inherit?

The incredible explosion of Christian institutions in western Medieval Europe during the 10-13th centuries, with their concomitant lands, taxes and other forms of income are testimony to the expansion of the influence and power of nobility through the power of placing sons (and daughters) in key roles. We should be clear first that primogeniture, although extensive, was not exclusive by any means.

We can point to this as a key structural problem in medieval society that underlay the investiture controversy of the 11th century which defined ecclesiastical and secular political relations for centuries to come. The conflict was ostensibly about the question of who could appoint bishops and abbots - the papacy or the the lay monarchy and nobility. This was triggered by the conflicted role of nobility in granting lands (with their taxes, tolls, productive means) to create monasteries and bishoprics: nobility, in creating these institutions, placed their non-inheriting sons, and sometimes daughters, in the leadership positions.

These positions such as bishops and abbots, under the peculiar medieval tie of land to power, wielded tremendous political and financial power, often greater than secular nobility. This power came into conflict with secular nobility over claims to land, income, taxes, and, broadly speaking, the rule over lives. At the same time, lay nobility was expressing tremendous influence and control over religious matters the papacy viewed as its own exclusive prerogative.

One scholar of this period, R I Moore, cast a humorous psychoanalytic sheen to this by calling the religio-political conflicts of 11-12th centuries 'the revenge of the second sons'.

The proliferation of bishoprics and the astonishing expansion of Cluniac and then Cistercian holdings, with abbeys and their daughter houses sucking up huge amounts of land, were driven by Christian piety, politics, economics and patrimony. A quick look at the origins of virtually every bishop, every abbot of this period, reveals the complete noble takeover of the local religious institution.

For a dramatic example of the route a non-inheriting child could take, have a look at the life of Bernard of Clairvaux. Third child of Burgundian nobility, he went from a canon school, to founding perhaps the most influential monastery ever, and becoming maker of popes, influencer of kings, supporter of crusades, and chief advocate for the creation of the Knights Templar. His 'power', although not defined in our minds as typical 'medieval feudal power' was greater than most nobility in shaping the fates and fortunes of millions.

Henry VIII was the second son of Henry VII and thus was not originally groomed to become king. Arthur (the heir apparent) was even betrothed to Catherine of Aragorn and married her in 1501 but died a year later at the age 15.

Prior to his brother's death Henry VIII spent a lot of his time preparing for a life of religion. Had his brother become king then Henry would likely have taken a position such as the Archbishop of Canterbury. However, Henry still received a lot of the education that his brother did with lessons in the usual grammar/theology/history/rhetoric etc. Unlike his brother though, he did not take part in as many royal duties in his early life.

Source: Samantha Ellsmore, David Rogerson, David Hudson (2001). The Early Tudors. UK: Hodder Education. 336.

http://www.amazon.co.uk/The-Early-Tudors-1485-1558-Advanced/dp/0719574846

The second son of a king would be in a very different place than the second son of a baron. The last time this question was raised, I actually traced the non-heir male offspring of French royalty through several centuries, and the answer is that they were, well, pretty well off. The Dukedom of Orleans was reserved for the oldest brother of the king. It did pass to their sons occasionally, and sometimes was reclaimed by the throne. The sons and grandsons of the French kings were generally dukes and counts.

If you're the non-heir male offspring of a minor lord, then you're in a bit of a bind (assuming you are in a primogeniture system where all the land goes to the first son, which wasn't universal). It was very common to pawn off extra sons onto the church (often paying a tithe to the church for the privilege), and a lot of the minor nobility became warriors, looking to make a name for themselves that way, either on the battlefield or in the tournament grounds. Just to pick one example out of hundreds, the 15th Century Italian condottieri were mostly landless noblemen looking to make some money and a name for themselves.

Clergy and the military were two prime options. The Teutonic order was an example of both.

Originally improvised as a first aid organization during the first crusades, it received an astounding amount of gifts from the Chrisitian nobility during the 12th-13th centuries, mostly in the Holy Roman Empire. Henceforth it was able to support many priests and knights to aid the crusader's efforts. Naturally, it recruited these people among the second and later sons of the generous noblemen... do ut des, as they say in the trade. This was beneficial in many ways: those sons could live a life in an appropriate style as was expected in the nobility, being priests they wouldn't produce legitimate heirs so they would be out of the way for the primal branch, and if all went well their succes would bring influence in the courts of the Holy Roman Empire. (And if something happened to the primal line, they could be fished up too.)

The Teutonic Order was organized in a number of bailiwicks; within these bailiwicks there were a number of commanderies. There was a hierarchy of commandeurs, the ones with the most seniority would get the wealthiest commanderie and its income. It would also be their place of residence if not on some mission (letters of the commandeurs to their rentmeester requesting money to be sent to them were quite common). As time progressed it became harder and hard to qualify for such a spot: progressively more noble ancestors were required (and a lot of networking, naturally).

After the crusades their efforts were directed against the heathen Prussians and other Baltic peoples. They conquered a large amount of territory along the Baltic coast, the present-day Baltic states and what would become Prussia in the future, with Mariënburg and Koningsberg as centre (now Malbork/Kaliningrad). They were eventually secularized and absorbed into Poland-Lithuania, but that's another story. The remaining possession of the Order were mostly situated in the Holy Roman Empire, and its efforts typically directed in wars against the Turks, until the Napoleonic conquests spelled an end to many feudal and religious instutions, including them.

Is there any truth to the saying that first sons take the throne, second sons take the military, and third sons take the clergy?

Primogeniture in England meant that second sons, no matter how powerful the family, did not inherit any land or titles if his elder brother(s) lived. Royal children were the one exception. As a consequence, most younger brothers of the nobility in England joined the military or clergy, though some also became craftsmen.

Here's a paper you may find interesting: The education and training of gentry sons in early modern England. It talks about the differences in education between siblings.

If this minor question is unacceptable, then I apologize, but wouldn't you be the son of an Earl? I didn't think there were counts in England.