"Ancient Times"

In the original Roman system, a 2D opening showed any 4441 hand with 17-20 hcp. The responses were the same as in their mini-Roman 2C opening: new suits were signoffs (opener runs if that suit is their singleton), and 2N is their strong inquiry (opener rebids their singleton).


"Modern Adjustments"

Today, most players play that a 2D opening shows any 4441 hand with 16-20 hcp. Some include 5440 hands, where the five carder is a minor. Also, some players have decided that after the inquiry: 2D - 2N; opener rebids in the suit below their singleton:
3C (diamond), 3D (heart), 3H (spade), 3S (club). 


"Blue Club"

The Blue Club system uses the 2D opening to show any 4441 hand with 17-24 hcp. 2S is an attempted signoff; 2N shows any bad six card suit; and 3 of a suit shows a good six carder (headed 
by at least AJ). 2H is used an inquiry; the rebids are:
 
2D - 2H; 
2S         : 17-20, either major singleton
2N         : 17-20, club singleton
3C         : 17-20, diamond singleton
3D/3H/3S/3N : 21-24, singleton in next suit (3N = clubs)

After the inquiry, responder can signoff at any time except in the relay suit (opener's short suit, except 2N over 2D-2H; 2S is considered the relay). 

For 17-20 hands, (except for the 2D-2H; 2S auction... see below)

The first relay ask asks for the range: 

• first step 17-18
• second step 19-20

The second relay asks for controls:

• first step 3
• second step 4 
• third step 5, etc.

The third relay asks for the number of queens:

• first step 0
• second step 1
• third step 2
  

For 21-24 hands, The first relay asks for the number of controls:

• first step 5
• second step 6
• third step 7, etc

The second relay asks for the number of queens.


Over the 2S rebid, 2N is used as a further inquiry:
2D - 2H;
2S - 2N;
3C         : 17-20, heart singleton. 3H is used as a further inquiry into the strength: 3S (17-28), 3N (19-20).
3D         : 17-18, spade singleton
3H         : 19-20, spade singleton

Over these response, the next relay asks for controls; then another relay asks for number of queens.