[maris oren]

Playing Short Clubs I can open with 12 HCP, minimum 5 card Major, 4 D or 2 C.
What are the rules for the 2/1 system?

[Stephen Tu]

There's no real difference with 2/1; 2/1 is based on 5 cd majors, and natural-ish wide ranging non-forcing opening 1 bids. Allowing short doubleton club openings is just a style choice that can be played whether or not you are playing 2/1 is GF

- 1D=4+, 1C=2+ is probably a minority treatment overall (but majority treatment in certain countries). A fair number of people open 1D with 4=4=3=2 shape, so diamonds is 4+ except that particular shape, and 1C is 3+.

- 12 HCP is for flat hands. With shapely hands people open lots of 11s and occasional 10s.

[Zelandakh]

Open the longest suit that matches the minimum length criteria.
With 2 5-card suits open the higher ranking.
With 4-4 in the minors, open 1♦ (or NT obviously).
With 2245 and honour strength within NT range, strongly consider opening 1NT.

You might come up with some exceptions later on (particularly with 5M-6m shape) but, to start with, these 4 basic rules will generally give you a good auction.

[Vampyr]

Zelandakh, on 2020-September-09, 11:04, said:

With 2245 and honour strength within NT range, strongly consider opening 1NT.

Does “strongly consider” mean “avoid at all costs”?

You could do it I guess if your doubletons are rich in honours. With 2-2 majors pretty much never.

[smerriman]

Vampyr, on 2020-September-09, 13:22, said:

Does “strongly consider” mean “avoid at all costs”?

You could do it I guess if your doubletons are rich in honours. With 2-2 majors pretty much never.

You're in a very small minority there. As Kit Woolsey put it:

Quote

If the hand is strong enough to reverse and is suit-oriented, I will open the 5-bagger. If not, I will open 1NT.

Virtually all 15-16 counts of that shape will be opening 1NT, because there is simply no other option.

[helene_t]

2/1 doesn't work so well with very light openings, so don't open mediocre 10-counts in 1st and 2nd seat with that system.

Otherwise, as others have said, the rules are the same as for other "natural" 5-card major with strong notrump (SEF, Standard American, Standard Hoog, Forum-D etc).