There are eight basic evaluation symbols used by chess annotators. They are as follows, given with their standard interpretations:
+- White is winning (has a decisive advantage)
+/- White has a clear advantage
+= White has a slight advantage
= The position is equal
=+ Black has a slight advantage
-/+ Black has a clear advantage
-+ Black has a decisive advantage
∞ The position is unclear
We look at a position and we want to know the truth. What's really happening here? What's the objective fact of the matter? That's part of what we expect a good annotator to tell us, and he or she does so by virtue of the symbols/evaluations above.
But something's strange here. If annotators are giving us the objective truth of the matter, then since there are only three possible results (not counting oddities like double forfeits), it seems we have five symbols too many! Objectively speaking, every position is either drawn and thus equal, winning for White, or winning for Black. Period.
How then should we understand slight and clear advantages and the unclear symbol? I think there are several, not mutually exclusive possibilities:
+=/=+ admits of at least three interpretation:
SA1. White/Black has a psychologically more comfortable position.
SA2. White/Black is more inaccuracies away from a loss than the opponent.
SA3. White/Black can expect to score about (say) 6 points out of 10 from this position when playing a true peer.
Both SA1 and SA3 are person-relative rather than objective. Re SA1, an omniscient chess player would presumably feel equally comfortable on either side of a position he, she or it can infallibly prove drawn. And re SA3, if the position is drawn giving omniscient peers, then there are no slight (dis)advantages for such players - which is what we'd expect. Finally, SA2 offers a semi-objective definition, in that it tries to find an in-principle, mind-independent feature underlying the advantage.
I think together, the three definitions offer a helpful way of understanding the notion of a slight advantage. Further, SA1 in particular is useful when deciding upon an opening repertoire. Maybe a grandmaster feels more comfortable on the White side of opening line X, but at the amateur level Black is more comfy. (Perhaps because Black has an initiative of a sort GMs know how to neutralize, but amateurs don't.)
Further reflection on these definitions and on situations where they might conflict could be of value, but let's move on to the notion of a clear advantage. An initial difficulty is that it's not clear (no pun intended) if it refers to an objectively winning position that's relatively likely to be drawn, or an objectively drawn position that's likely to be won. So we'll have a further bifurcation:
+/- and -/+ can be understood in at least the following ways:
CA1. White/Black is winning, but technical precision, a high degree of accuracy, or some inaccuracies from the opponent, must be present before the advantage is best classified as decisive.
CA2. The position is drawn, but Black/White is only a small error away from a losing position.
CA3. White/Black is likely to score about (say) 8 out of 10 against a peer.
For those of us used to checking with our analytical engines, we're used to seeing a numerical range affixed to the clear advantage sign, and there's a pretty substantial range between a slight and a decisive advantage. Perhaps, then, we might think of some "clear advantages" as draws that can become wins, and others as wins that can become draws. Thus CA1 reflects what we could call a "large clear advantage" and CA2 a "slight clear advantage". Maybe the best way to capture the essence of the term, then, is the following:
CA4. White/Black has an advantage such that (1) the opponent's position is distinctly undesirable, but (2) not yet obviously lost. (If I may be permitted a semi-joke, it's roughly like Black's position after 1.e4 e5 2.Nf3 f5?)
On now to "unclear". This fine term is the most polysemous of all:
U1. The annotator has no idea what the heck is going on.
U2. The annotator is too lazy to figure out what's going on.
U3. The annotator may know what's going on, but sure isn't going to tell you. (This is a common trick in opening annotations.)
U4. The annotator knows that one side or the other is in trouble, but since the weaker side has a vaporous initiative and the annotator is promoting a book/article/video/CD/DVD supporting the crappy opening played by the weaker side, he inserts the unclear symbol before the true evaluation is obvious. ((Mildly) exaggerated examples: 1.e4 e5 2.Nf3 f5?∞; 1.e4 e6 2.c4 d5 3.cxd5 exd5 4.Qb3?!/? dxe4 5.Bc4∞)
U5. The annotator thinks the position can be evaluated in one of the other ways listed, but the position is too volatile for him to have a high degree of confidence in that alternative evaluation.
U6. "Unclear" generally connotes the annotator's inability to express a honest preference for one side or the other, but occasionally it is used as a modifier of another evaluative term. (For example: "After Kasparov's move the win is obvious, while the showy 36.Be6 Nxe6 37.dxe6 Rxf2 was unclear." The annotator might mean that the evaluation is totally up in the air, but he might also mean that it's only unclear that Kasparov is still winning in the sideline.)
To summarize and apply our discussion of this last term, when you see "unclear" in a regular opening book, beware and do your own analysis. In an opening book advocating some "underrated" (read: garbage) opening, assume the author is, shall we say, optimistic, and assume the line is bad in the absence of your own analysis to the contrary.
In short, beware.
P.S. My digs at the Latvian Gambit were not intended as critiques of any particular author or book.
P.P.S. A future post may examine the even wackier world of exclams and question marks; there too, distinctions can be made between orthodox textbook definitions and the various ways they are used in the real world.