I'll disagree with rak slightly. If partner plays 4th

on 3rd

, there are 4 scenarios that make sense:
a) partner started with exactly 3 cards.
b) partner started with 8...MAYBE 7...and has no more points to play.
c) it's the point that saves the meld/bid
d) I'm declarer, and I sandbagged the suit originally
d) first. This is somewhat unlikely, as it feels like it requires dummy hasn't been in; otherwise, he'd cash his lone

(we hope). But as declarer, I'm suggesting extra tricks in that side suit...and it should be several. Why am I starting my side suit from the top with the aces, rather than leading low, if I have, say,





? Dummy has to consider what the defense has done...have both apparently cashed? What's declarer's length in the 2 side suits, and critically, has he played several rounds of trump, trying for at least a partial trump strip? But here, if dummy plays his 4th ace, it's an unblock, counting on declarer to have some

's.
c) is also relatively uncommon, and much more likely on defense.
a) implies that partner will be ruffing, which gets him in. So it's a signal in an unusual sense. Partner SHOULD NOT play 4th

with a 4, 5, or 6 card initial holding, as this will cost a trick too often. Remember: a trick is 2.5 points...and the 3rd and 4th ace tricks catch opponents' point cards more often.
b) is the tricky case...7 or 8 cards. Generally, the only time (even with an 8 card suit) that I'll play 4th on 3rd, is when:
1. I think we need the point. Case: I'm dummy and I only had 4-6 meld. Partner needs to pull well over a split deck to make.
2. I have no great desire to be put on lead. That means my side-suit aces are in 4 or 5 card suits...and I don't have many.
Even then...I'm not sure I would. Reverse the situation and consider the position from partner's perspective. When I play 4th

on 3rd

, he's *probably* going to assume situation a)...and therefore, continuing the suit would get me in. IF he's reading the opponents' cards, he may work it out...figure, you have an 8 card suit with AKK and 5 non-points. The opponents have to be playing mostly points. So *if I trust partner*, my 4th

*can be read* as suggesting the long suit.
This *could* happen with 7 cards, but obviously the length difference, even tho it's only 1 card, is very significant. As noted, opponents' cards may help. Say the first 2 leads draw:

,

,

,

,

,

,
then the 3rd ace:

,

, ???
The opponents won't play points voluntarily. At this point, 6 of the 8 non-ace points have been played, and 4th seat is either going to play #7, or ruff. If he's ruffing, I actually don't want to play my

...but it can be forced out of me even if I don't. If he's not ruffing, then with 7 points out...it's a virtual certainty that one of the opponents will ruff the next round, so it's NOT a trick. Save the point.