Status of a sandhi "rule" for 2nd tone

Question

I came across a sandhi rule - described here (point number 6):

A 2nd tone preceded by a 1st or 2nd tone and followed by another tone becomes a 1st tone

I never heard of this rule before so I was wondering if it is well known. I found one other reference for it here: Is there a tone sandhi for 2nd-tone-2nd-tone names?, where it's described differently:

In a trisyllabic expression, if (a) the first syllable is T1 or T2, (b) the middle syllable is T2, and (c) the final syllable is not weak, then the middle T2 can change to T1 in conversational speed.

Is this rule commonly known? Is it commonly accepted? Are there any other references for it?

Thanks

Edit, 2013-05-23 Back in 2018 I wrote to the author of the article to ask for more information about the rule he mentioned. He told me he'd written it based on some memories from a Chinese language class and a textbook; he decided to edit his article by removing that particular rule.

Answer 1

I asked the author of the blog eastasiastudent.net about it and he told me it was probably a mistake. Given the comments from @fefe and @JasonSwift, my conclusion is that the author of the first link was trying to describe a real phenomenon in spoken Chinese (i.e. the phenomenon described in the accepted answer of the referenced SE question) but made a few mistakes in the description.

Answer 2

It seems there are indeed sandhi rules for second and fourth tones, in certain situations. However these rules aren't very well known.

These rules are described in wikipedia. They can be resumed as follows.

1. Tone 2 becomes higher and changes its direction, approaching the tone 1 pitch contour, when put between tone 1 or 2 and any other full tone.
2. The preceding rule also applies to the rising tone induced by the tone 3 sandhi rule.
3. Tone 4 becomes lower and flatter, but still slightly falling, akin to Cantonese tone 3, when put between tone 3 or 4 and tone 1 or 4.
4. The above rules hold for conversational speech. The exact pitch contour produced in rules 1 and 2 depends somewhat on the speed of the speech; whereas the pitch contour for the last rule is less dependent on the speed.

I recommend looking at the wikipedia reference as it gives more detail and provides examples.

I think these rules aren't very well known because they are fairly complicated to describe and remember, and could be hard to hear as they only hold for conversational speech. They may only be useful for intermediate/advanced students, and even for such students it's probably better to learn these rules unconsciously/naturally by hearing native speakers and talking with them.

These rules are best described as optional, as opposed to the well-known obligatory rules, such as the consecutive third tones rule. See also this article by Olle Linge, it does a good job on explaining why this phenomenon occurs, and how students can use this knowledge.

There are several other CSE questions on this topic:

• how-do-natives-pronounce-the-2nd-tone-in-regular-conversation-pitch-levels-35
• how-to-pronounce-words-with-double-fourth-tones
• is-there-a-tone-sandhi-for-2nd-tone-2nd-tone-names