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I don't think can/may/must can be found there either (maybe I am wrong) but that is the way it works.

There are 6 degrees of freedom.

If it CAN constrain a degree of freedom, and it MAY (because it hasn't been constrained yet) then it must.

At a guess since I don't have your part, "A" looks like the center bore. CAN it constrain 2 degrees of rotation? YEP, none have been constrained yet. CAN it constrain two axial degrees of freedom? YEP, none have been constrained yet, so "A" constrains 4 of the 6 degrees of freedom.

Next is "B", the end of the part. CAN it constrain 2 degrees of rotation? Yep, but may it? Nope, both have been constrained already. CAN it constrain 1 axial degree of freedom? YEP, the one it can constrain hasn't been constrained yet. So "B" does 1 axial degree of freedom. We are now at 5 DOF.

That leaves "C", CAN it constrain 1 degree of rotation? YEP, and it hasn't been constrained yet, so it must. CAN it constrain any axial degrees of freedom? YEP, but it can NOT as all of them have already been constrained. So, it does 1 degree, giving us the 6 of 6 that we need.

I don't think can/may/must can be found there either (maybe I am wrong) but that is the way it works.

There are 6 degrees of freedom.

If it CAN constrain a degree of freedom, and it MAY (because it hasn't been constrained yet) then it must.

At a guess since I don't have your part, "A" looks like the center bore. CAN it constrain 2 degrees of rotation? YEP, none have been constrained yet. CAN it constrain two axial degrees of freedom? YEP, none have been constrained yet, so "A" constrains 4 of the 6 degrees of freedom.

Next is "B", the end of the part. CAN it constrain 2 degrees of rotation? Yep, but may it? Nope, both have been constrained already. CAN it constrain 1 axial degree of freedom? YEP, the one it can constrain hasn't been constrained yet. So "B" does 1 axial degree of freedom. We are now at 5 DOF.

That leaves "C", CAN it constrain 1 degree of rotation? YEP, and it hasn't been constrained yet, so it must. CAN it constrain any axial degrees of freedom? YEP, but it can NOT as all of them have already been constrained. So, it does 1 degree, giving us the 6 of 6 that we need.

I'm impressed you whipped that all out in under 13 minutes.

Thanks for the very thorough response. I read this too but in less clear terms than you have spelled out. So if one degree of freedom constrains more than should it be first?

also:

"you whipped that all out in under 13 minutes"