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Each rule ended up creating a typeform, which simply put means that while each rule was intended to fairly handicap all vessels, the designers ending up discovering traits that were favoured by that particular rule. Here is a summary of the type forms (in my opinion),
CCA - American based rule. Relatively narrow hulls (compared to today) and shallow (for the day) with fuller ends and fairly steep and wide counter. The Cal 40 is a good example of a CCA type design.
RORC - English based rule. Beamier hulls with a more wineglass section and narrower sterns than CCA. Think of earlier S&S type designs.
IOR - Developed as a melding of the CCA and RORC. Sail area measurements more closely followed the CCA while hull measurements were more adapted from the RORC. Early type forms were more S&S type, quickly followed in the early 70's by the Peterson/Holland concept of short pintail sterns. Deep forefoot and U-shaped bow sections. Of course the Peterson trapezoidal keel became a feature of mid 70's IOR boats. Just when everyone thought the IOR promoted heavy, overcanvassed, downwind rollers - along came the Kiwis in the mid-late 70's with their big butt lightweight fractionals, which changed the landscape. Turns out that IOR Mark II actually encouraged lightweight fractionals rather than heavy mastheads, so the IOR quickly moved to effectively ban those types. Later (80's) designs were more moderate. Lightish weight designs with widish sterns and fractional rigs (up to the size where it was feasible)
MORC - This rule was limited to boats 30 feet and under. Short overhangs, wide sterns, moderate displacement and masthead rigs. Think Santana 30/30 or Pinnacle 30 as MORC typeforms.
IMS - Lightish designs with wide butts, short overhangs, plumb bows, fractional rigs, blended in keel bulbs (apparently IMS favoured this style as opposed to the distinctly separate bulb you see on most modern sport boats)
Not sure about the more recent rules - gave up caring about measurement rules a while ago.