Landy is the first bridge

Contract bridge

Contract bridge, usually known simply as bridge, is a trick-taking card game using a standard deck of 52 playing cards played by four players in two competing partnerships with partners sitting opposite each other around a small table...

 conventional defense against over opponent's 1NT opening and one of the first conventions ever introduced. It was invented by Alvin Landy. Over 1NT, the overcall of 2 shows both major suit

Major suit

In the card game contract bridge, the major suits are spades and hearts . The major suits are of prime importance for tactics and scoring as they outrank the minor suits while bidding and also outscore them...

s, while all other bids are natural. Requirements for the overcall vary from partnership to partnership: some require 5-5, some 5-4, and yet others only 4-4 (provided the overall strength is sufficient). The partner can take a preference to either major or make a non-forcing bid of a suit; 2NT is used as a forcing query.

Multi-Landy

A particular popular modification in the Netherlands

Netherlands

The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 is Multi-Landy, a combination of Landy, the multi 2 diamonds

Multi 2 diamonds

Multi or Multi 2 diamonds is contract bridge convention whereby the opening bid of 2 shows multiple types of hands, which always includes a weak two bid in either major suit, while the additional meaning may be a strong balanced hand , or a 20-22 three suiter...

 convention and the Muiderberg convention. In this modification, the 2 overcall is the same as in Landy, the 2 overcall shows a 6-card major suit, and 2 or 2 overcall shows a 5-card in that major suit and at least a 4-card in a minor suit

Minor suit

In contract bridge the minor suits are diamonds and clubs . They are given that name because contracts made in those suits score less than contracts made in the major suits , and they rank lower in bidding. In particular, one can make game with a bid of 4 in a major suit, while a bid of 5 is...

.

Woolsey or Robinson

There is a variant known as either Woolsey or Robinson named for Kit Woolsey and Steve Robinson who developed this system. Woolsey uses the same responses as Multi-Landy (above) but also includes a pinpoint double. A double would show a 4-card major and a 5-card or longer minor, a constructive 6-card or longer single-suited minor, or a hand of 19 high card points or more.
After 2, advancer would bid 2 to show equal length in the majors and ask overcaller to bid their better suit.
After 2, advancer would respond 2 to ask overcaller to pass or correct. Advancer would bid 2 showing non-forcing values in spades but invitational values for hearts. Advancer would bid 2NT with a forcing hand asking for further description of overcallers hand. Advancer would bid 3 with invitational values in both majors. 3 or 4 are pass or correct at that level.