why are there 4*numberDOF elements in the disp array?

[docmao]

int
Node::createDisp(void)
{
// trial , committed, incr = (committed-trial)
disp = new double[4*numberDOF];
.......
}


I want to know what means of the disp elements?

[docmao]

In my thought, it should be numberDOF elements in the disp array. But it seems the disp array keeps more displacements information in the Node class. So, what do they means?

[fmk]

the array keeps current trial, last committed, the incremental disp(trial - commit), and the delta incr disp (current trial-last trial)