hi boo, well my children with my second wife are actually, from birth, only Sha (8) and Rivka (nearly 6 months). We have Ciaran who we didn't have custody of until he was 2 as he is an ex girlfiend of mine's little boy who sadly died, so my experience of this system from birth is limited to Sha,as all of my other children have been from my previous marriage. My wife used this with her son, now in his 20's and apparently it worked for her ( it's her system). My brother's girlfriend is also using it on her little girl Adele who is nearly 3 now and it has worked pretty much along the same lines with Adele.Ciaran would be at about the same stage as your 21month old when we had him and he settled into the idea quite quickly but as Le Chat says just because they understand doesn't mean they naturally obey and initially when they realise they can ignore you, you need to be VERY determined and repeat the eye contact,NO, re-inforcement over and over, but don't shout. I don't see why this will not work with your little one as she's at the same stage as Ciaran was and we had very few tantrums with him, and it really does make for a very calm and happy house.Rivka is just at the rolling, crawling everywhere stage and getting into things she shouldn't and is starting to recognise NO and occasionally act upon it, so I'll keep you posted.As far as I'm aware there are no websites about this, as it's not widely recognised that kids that young can meaningfully take on board such complexities, but if anyone knows of any preaching similarly I'd be interested in them too.It's nice that Le chat recognised that her 9 month old was able to understand in a similar way as the amount of people who just will not accept it's possible is incredible, but I honestly don't think it's unique to the kids we have and know, I think in some degree or another it's across the spectrum with obvious subtle differences depending on the child and indeed the parents.Hope this helps although it's a bit