[Wainfleet]

I have come across the term "courtesy raise' several times recently. What is the difference between this and a straightforward simple raise?

[HardVector]

It is a raise that you make not because you think you can make a game or slam, but because the information that you have a fit makes your partner's hand better. For instance, the auction has gone p-p-1c-p-1h-p. Your partner is a passed hand, you've made an opening in clubs and partner has bid a heart suit. You have;
Kxx QJxx xx AQxx. A pretty minimal hand and considering that partner couldn't open, game seems unlikely. Passing 1 heart is allowed here but holding a good fit for partner, you should make a "courtesy" raise to let your partner know you have a fit. It may be they have something like; Ax T9xxxx Axxx x and now you can get to the heart game. Notice, this only has 20 combined hcp, but it's a good game.

It is also applicable when partner has shown a big hand. The knowledge that a fit exists may make their hand better. For instance: (1s)-X-(p)-2d-(p)-2h-(p). They have opened, your partner doubled then showed a very good hand with a heart suit. You have xxxx xxx Qxxxx x. A truly miserable hand, but you do have a heart fit as well as some value in being able to ruff clubs. A pass is not out of the question, but you should consider a raise in case the knowledge that you have a fit is all partner needs to know.

[Stephen Tu]

Usually this term is used on auctions like:
(uncontested) 1♥-1nt-2♣-3♣

Where 3♣ can be just like a good 8 count and 5+ clubs. Where partner still has a wide range (here, partner can have up to 18 or so), and you are mainly keeping the auction alive just in case he has the top end and you have game, but at some risk of going down opposite the lower end. You are raising more as a "courtesy" to partner to keep the auction alive in case he is quite strong rather than as a invite to game.

Here with a real, strong invitational raise, you would bid 2♠, the "impossible spade" bid (with real spades you'd bid 1S over 1H), the way most people play. This trick is obviously not available when partner opens 1♠, so on those auctions the raise becomes more fuzzy and wider ranging.

[DavidKok]

Another standard situation for a courtesy raise is if partner overcalled instead of opened - (1♦)-1♥-(2♣)-?. Here most pairs would play 2♥ as something like 4-8 points (even without the 3rd hand bid partner is limited by a failure to double and then bid hearts) and 2♦ as forward-going.

[paulg]

I play courtesy raises after a response to a takeout double. The key facet of the courtesy raise is that it guarantees the fit with no extra values.

For example

(1D) Dbl (Pass) 1H
(Pass) 2H just guarantees four hearts and a minimum takeout double.

It is a hand that traditionally would have passed a one-level response. If the doubler has a stronger hand with fit then they start with a cue bid.

This style allows responder to stay lower when they have the values for a jump response but perhaps only a 4-card major.

[mikeh]

Stephen nailed it. I was going to give precisely the example he chose.

The other examples are not, to me, courtesy raises. They are, if you play that style, more an effort at preempting the possibly balancing opps.

[nige1]

All of the above. Also 2♣ - 2♦; - 2♠ - 2NT; 3♠ - 4♠ with e.g. ♠ 7 5 ♥ 8 4 2 ♦ 9 7 5 3 ♣ 8 6 4 2