Why was my new board twisted? Because it had been cut from a 'tree' species unknown, age unknown, season unknown but probably pine, spruce or fir. And then thrown in the sea. When boards dry out, maybe especially if kiln-dried to speed things up, the outside sap-wood will dry out at a different speed to the central, harder, more lignified heart-wood warping results to minimise the tension in the fibres. The knots come, as you know, from the foundation of side-branches sprouting from the main millable trunk. Saw-millers have known this since first a board was cut. They want to get as much usable lumber from each log that passes through their hands. In the old, local, pre-globalisation days, trees would be felled for a particular purpose in mind, cognisant of the fact that when planks, especially sap-wood planks, dry out they will have a strong tendency to straighten the rings. Amongst much else that means that floor boards have a natural up-side and a under-side. If you nail it upside down, the centre will 'sag' edges will finish 'proud' and put splinters in your socks. Hogging planks are fine because the edges sit down. Of course, with such long-established knowledge the language will be weirdly medieval.
super-wonderful page on how to mill a log so that it is fir (sic) for purpose. Obviously if you need a mighty timber to support an upstairs or a fire place then you'll cut a whole piece and use the super-bendy off-cuts somewhere less structurally crucial. And you have to have something to burn in the fireplace too. This all took me back to 1972, when I was wrapping up the school-days under the care and attention of Mr Wilkinson my inspirational biology teacher. My biology notes accumulated in 4 hard-backed exercise books which I've managed to hold on to. For reasons best know to himself, probably because he was really interested, he spend a couple of lessons talking about forestry, coppicing and timber. That was never going to 'come up' on the exams but it stuck to me anyway.
B for Boxing-the-heart is a compromise, as are all the cuts labelled in the previous picture. The thing about the generic cuts, like 4x2 or 3x2 is that they are good-enough for a wide variety of situations and so long as you keep the timbers dry-enough and airy-enough any old softwood or any old species will do. Even if you make the ceilings 2.4m high because that's half the standard length of sawn timber in the yard, you'll still have off-cuts and you can usually arrange it to discard the cruddy end of some of the timber.