SIZE()

[Angus]

M1:=5; //-->[[5]] *****
M1+M1; //-->[[10]] not [25] (or [10] )
DOT(M1,M1); //--> Error: Invalid dimension

M1:=[5]; //-->[5] *****
M1+M1; //-->[10];
DOT(M1,M1); //25


and finally:

M1:=[1 2];
M2:=[[1 2]];
M3:=TRN(M2); //!!!!
M1+M2; //Error: Invalid dimension
M1+M3; //Error: Invalid dimension



M1:=[1 2 3];
TRN(M1); //-->> [[1
2
3]] //transose of vector.....now matrix.

I have heard of line or column vectors in former days.
you see?

What makes a vector different from a one dimensional matrix?

For the prime it is. Just because of internal types? They are even created by the same Matrix-Tool.
I tried it a last time. I think you won't see my point again (or at least it was not referred), but I wanted to try it.