Landy is an extremely simple defense against 1NT openings. Here goes:

(1NT) — 2:    Shows at least 5-4 in the majors.

The 2, 2, and 2 overcalls retain their traditional meanings, and a double is for penalty.
        After Landy, responder will typically just bid his better major:

(1NT) — 2 — (Pass) — 2:    Signoff in hearts.
2:    Signoff in spades.

However, he could conceivably pass with a very long club suit, or bid 2 to show a diamond suit.
        NOTE:    Some pairs like to play the 2 response as showing equal length in the majors. The overcaller should then choose between 2 and 2 for the final contract.


        Finding Game.    Here is one possible system for exploring game after Landy:

(1NT) — 2 — (Pass) — 2NT:    Shows interest in game (not forcing).
3:    Artificial and game-forcing.
(Overcaller should bid his better major).
3, 3:   Invites game. 
3NT, 4, 4:  Signoff in game.