Most grantor trusts are established between a husband and a wife with the express condition that either can revoke it during lifetime, but neither can revoke it after the death of the other. Allowing one grantor to "withdraw" money without writing a check would be allowing a revocation in part without notice to the other grantor.
I've sat through a number of H & W negotiations, particularly when both had children from a prior marriage, when a move by one to take money out of the trust without the knowledge of the other would have resulted in bloodshed.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.