The way I think of the system, it's not necessarily about the shapes and features that can be easily outlined – it's about all the types of motions our bodies and faces might be implying. The sequence of IDs is a continuous chain of one type of motion transitioning into another.
(The illustrations of individual faces are from a site called OURFASHIONGARDEN. All credits to them.)
The order is as follows:
SG implies compression, the body can be divided into concentrically compressed articles;
SN fights with compression, indiviual articles are trying to fuse together, but the body is still compressed as a whole;
R breaks free, it implies expansion (it needs frills and ruffles);
SC starts focusing the outward forces into one direction;
SD is almost focused into verticality, but the remaining "outwardness" retains mass at certain places (= the implied flow creates mighty meanders and reaches the ground in full strength);
TR has lost all sense of outwardness, the flow is emptying itself (= the meanders are getting smaller and the flow can't even reach the ground anymore, it's too delicate);
DC reframes the straightening process as empowering, it feels like solidification;
D is perfectly straight by now, but just not solid yet (stiff fabrics are okay, but they should hang down from the shoulders);
FG gets full-on solid and breaks into fragments (the horizontal cracks start hinting a sense of width);
and in FN, the width is explicit – the advancing fragmentation has made the body so articulate that it can even handle unshapely oversized clothes.
Going all the way from SG to FN is like going from micro to macro. It's a full circle, but not really, if you know what I mean… An upward spiral (a single-coil spring).
It's important to understand that I'm only talking about the continuity of the SENSE that we make (or should make) of individual IDs – I'm not implying any real chronological development! To connect it all in any other way, so that it doesn't feel like an accidental sequence of shapes, would mean to compromise on how tight one's grasp is on individual IDs.
People often mistake soft naturals for romantics. They say that some FGs are influenced by dramatics and some by flamboyant naturals. This might be the reason – they lie next to each other.
I'm not sure what David Kibbe's opinion is, but it caught my attention that he assigned the same attribute (flamboyant) to gamines and naturals. Flamboyant naturals might have a long vertical, but it is manifested differently in them than in dramatics. They do look like they're somewhat bound to the ground, but the vertical fluidity is not there. They're quite static, actually, just like the gamines, and thus just as easily accessorizable (except only the accessories need to be bigger).
By the way, you may have noticed that the four main principles resemble the four natural elements:
compression represents the air – its dimensionless particles;
expansion represents fire – a spontaneous spark coming out of nowhere creating a sense of spatiality;
continuity represents water – the intuition diverting fire's energy in search of meaning;
and fragmentation represents the earth – the sense of congestion emerging as all the connections fill up the space.