Quote:
Originally Posted by
Maxilocks 
Because it's not about your parents being able to use magic, but being of pure magical blood, meaning that both parents have to be born to pureblood parents, not just know magic. That's why wizards and witches that believe in blood superiority are so smug - they can boast that they've never, never at all, had any muggles in their lineage.
Frankly, I find it really, really easy to decide if a character is a pureblood or halfblood. Rowling's rules about it have always seemed crystal clear to me. For meee... No muggles in the lineage? That's a pureblood. Muggle-born wizards and witches in the lineage? That's a halfblood. Both parents muggles that can't use magic [and aren't squibs]? Then the child is a muggle-born witch or wizard. I believe that to be canon. ^___^
I agree with you completely and as for the squib thing, If they're from a pureblood family, to me, if they had offspring with a pureblood (or pureblood squib) then those children would be pureblood (or... squibs), but if they had children with halfbloods or muggleborns then their children would be half.
I don't know WHERE I originally got it from, (possibly a fic somewhere?) But one 'rule' that I found/find useful is the idea, as Steely said, with it being measured by the blood of the grandparents. So a child can look at their parents and their grandparents, and if all are of magical blood, then that child is pureblood.
(of course I'm sure 'genuine' purebloods might look down on those self proclaimed types XD)
As an aside:
It'd be interesting if it could be measured by a dominant/recessive gene since that'd mean in some cases you might get siblings that are half and pure, just like you can get different eye colours or even skin tones.