In our version of Polish Club, the 1♦ relay denies invitational values. This
relay contains all hands without another suitable bid.
Strength
Distribution
0–6
Any
7–9
0–3♠, 0–3♥, probably UNBAL
15–17
(32)44
Traditionally, 2♦ is game-forcing and 2NT is 21–23. I find 2♦ low enough to
accomodate 21+. Then, 2NT takes care of 18–20 natural clubs, alleviating
burden of 2♣. The side effect is that most (xx)45 are better opened 1♦.
The 2♦ rebid shows a nearly balanced hand with 21–23 HCP or any game force. If
you can force to game, bid as if partner opened a natural 2NT. Otherwise, try
to sign off at the 2-level.