Inverted minor is the simple raise of partner's 1♣ to 2♣ or 1♦ to 2♦ - it shows at least an invitational hand (further details below). Traditionally this was played as just a regular raise - say 6-10 points and 3-4 card support. That is now rare, whereas inverted Minor is very popular.
What is it?
As mentioned it is the raise of 1♣ to 2♣ or 1♦ to 2♦.
What does it show?
- 4+ card support (if you play a short club, then 1♣ - 2♣ should show 5+ clubs)
- 10+ points (invitational +)
- No 4 card major - with a 4 card major it is standard to just start by bidding it at the 1 level.
What are the continuations?
Rule: Inverted minor is forcing to at least 3 of the minor. If both players have a minimal hand, they can each pass 3 of a minor. If either player has extra points, they can bid on.
An exceedingly simple but very effective way to play is that all bids are natural.
Lets look at an example auction
Example 1
This is an easy 2NT bid - you have points in all the other suits and want to let partner know that you're balanced and suitable for No trump. In general - with a balanced hand, it will be a good idea to bid 2NT to tell partner you are balanced. If, however, you have an unbalanced hand, you may show that differently, for example
Example 2
Example 3
Many pairs show shortage in this auction by jumping a level, for example
Here 3♦ is a shortage, because it was a jump (where 2♦ would've been natural and forcing).
Example 4
We should always rebid 3 of the minor with a minimum hand, let partner know that. Responder can do this as well, for example.
Here we have started off with 2♦, showing 10+ points and 4 card diamonds. We now bid 3♦ saying - that is all we have!
Additional things to think about
What do you do with a simple raise now?
The common thing to do is to play 1♦ - 3♦ as a simple raise with 4 cards, say 6-9 points. With 3 card diamonds, don't raise diamonds, just bid 1NT (or a major of course, if you have one).
Advantages
This is a very good way for handling hands with a minor that are invitational plus strength. It also has the advantage of being very natural, easy to remember.
Disadvantages
Some people who are used to playing 1♣ - 2♣ as a simple raise, may forget the system. It is sometimes hard to fight a reflex built over years, but it can be done!
There are not many other disadvantages other than the bidding being forced to the 3 level, but usually that will be okay as you have a fit and enough strength.
Recommendation
I like inverted minor and find that most experienced pairs are using it. It handles these type of hands very well, easily creating a force and describing the hand accurately and naturally. I recommend it to any pair, but as always first and foremost what you are comfortable with and what you prefer stylistically should be the priority - if you like simple minor raises, its fine to stick with that and still get good results. Bidding is more about comfort and confidence than optimisation, at all levels.
Read next: Bergen Raises: A Bridge Bidding Convention · Splinter Bids in Bridge: How and When to Use Them · Better minor or short club?