What, Exactly, Are Bloodlines?

Surely if mages are so powerful they would have conquered the world by now? Well, perhaps not, considerin that they are lacking in one thing that the human race has become famous for - Numbers. 

The mage community is growing smaller and smaller with each passing generation as less and less people choose to practice the magical sciences and many of those who do don't contain the inbound potential to wield prana properly. Every human being has prana in their blood but very few people have the activated magic circuits required to use said prana for anything other than sustaining their own existence.

Bloodlines are simple - The longer your magical lineage, the more magic circuits in your blood and the higher your potential magical transference and the larger the result. Though mages don't need a large blood line to be powerful, with mages like Waver working in earlier grail wars, they tend to be very limited with what they're actually capable of and are thus forced to work off of less impressive schools of magic, such as alchemy.

The unfortunate thing about this is that the older sibling always becomes the heir to a family's lineage, whether or not they have the most magical potential. Because of this, those in magi circles commonly give their children away for adoption into other families to allow them to have a future and to avoid other noble clans from dying out. Because of this, it is often confusing exactly how long a line is, when the son of "Lord El Maloi I" can become "Lord Matou IV" just through adoption. 

That being said, the longer the bloodline, the more "noble" the clan. The Matous, the Einzberns and the Tohsakas are widely considered to be the creme de la creme simply because of how long they have been around and thus how much power is given to their actual heirs. It does not matter how they act, nor how polite or sociable they actually are, a mage's status rises with his power and these people were born with the proverbial silver spoon in their mouth in that department. 

That being said, since not every other noble family requires a new heir and not every family is cold enough to send away their beloved children, it isn't unheard of for mage families to have between two and four members of each generation, though the business of their parents lives means that families with four children and above either have had a lot of luck with triplets or have been ignoring their studies.