Confusion over ip tcp adjust-mss and ip mtu.

Experts,

Why we need both commands in a router?

If we confgiure ip mtu 1492 , doesn't it mean that MSS=1492-40=1452.

Then why we need ip tcp adjust-mss?

Also if we configure ip tcp adjust-mss 1450, does that ip MTU get auto adjusted to 1490.

Also why we need PMTUD, if we are negotiating MSS in TCP sync packet? Im totally confused..

Krishna