Overcalls after a 1NT opening

The main overcalls after a 1NT opening are:

a suit normally with 6 cards
2 clubs (Landy convention) with the majors at least 5-4
2NT calling for the minors with preferably 6-5
Double showing a long minor and a major of at least 4 cards
Preempts with suits of 7 or 8 cards.

To overcall in a suit you need

a 6-card suit
and an unbalanced distribution.

The hand's distribution is more important than the number of points HCP.

S	W	N	E

Problem E31484 PASS Your 13HCP are hardly worth anything!

2 clubs Landy
This is a very efficient and economic overcall promising

at least 5-4 in the majors
and opening points

The bidding after 2 clubs:

2 diamonds shows a weak hand asking Partner to choose his best major
3 hearts or 3 spades proposes game

S	W	N	E

Problem E31504 2 clubs Landy to announce your 2 major suits

2NT calling for the minors
2NT shows a hand with 10 or so HCP and, preferably, 6-5 in the minors (the 1NT opener is more likely to have strong minors than majors...)
In response to this overcall, Partner bids his best minor.
DOUBLE : showing a long minor and a major of at least 4 cards. This overcall is much more frequent than the Landy or 2NT. See the corresponding information note.