<p>真的是个好问题，我原来总结过一些规律，但可能不靠谱。我感觉是根前后字（组合成的那个字）有关。
比如：</p>
<blockquote>
<p>一二三四，读1声</p>
<p>一骑绝尘，读2声 （如果骑读作ji4的话）</p>
<p>一马当先，读4声</p>
</blockquote>
<p>我发现，除了&quot;一二三四&quot;，如果&quot;一&quot;后面跟4声的词，&quot;一&quot;应该读2声，除了上面的例子还有诸如：</p>
<blockquote>
<p>一箭双雕，一泄千里，一世英明……</p>
</blockquote>
<p>如果&quot;一&quot;后面是1声、2声、或3声，&quot;一&quot;应该读4声，诸如：</p>
<blockquote>
<p>一帆风顺、一尘不染、一鼓作气、一往无前……</p>
</blockquote>
<p>也不知道准不准，欢迎提出反例。</p>

CC BY-SA 4.0
2020-10-19T02:42:30.200Z
Older versions of the dictionary will say 一骑`yi2ji4`, but newer versions say `yi4qi2`, in PRC.
River
CC BY-SA 4.0
2020-10-19T03:14:32.480Z
@River，确实，如果读骑读ji4，一应该读2声，如果骑读作qi2，一应该读4声。
Zhang
CC BY-SA 4.0
2020-10-19T01:57:29.147Z
2020-10-19T03:19:50.720Z
2020-10-19T03:19:50.720Z
Zhang
<p>The rule of thumb is the following.</p>
<p>When indexing it is <code>yi1</code>. For example</p>
<ul>
<li>一楼 (first floor)</li>
<li>一声 (first tone)</li>
<li>第一 (the first)</li>
<li>一次世界大战 (World War I)</li>
<li>一来 (firstly)</li>
</ul>
<p>When counting （the same rule as 不）</p>
<ul>
<li>it is <code>yi4</code> when followed by 1, 2, 3 tone
<ul>
<li>一呼百应</li>
<li>一毛不拔</li>
<li>一百</li>
<li>一扫而光</li>
</ul>
</li>
<li>it is <code>yi2</code> when followed by 4 tone
<ul>
<li>一蹴而就</li>
<li>一气呵成</li>
</ul>
</li>
</ul>
<p>For others, mostly <code>yi1</code>. For example</p>
<ul>
<li>统一</li>
<li>均一</li>
<li>敬一丹（name）</li>
</ul>
<p>As a comparison, 二 can be indexing or counting, but 两 is only counting.</p>
<p>PS: Most of the words in the Wikipedia list are right, but there are clear errors. For example &quot;一<code>yi4</code>夫一<code>yi4</code>妻&quot; appeared in 2 lists. And, it should be &quot;一<code>yi4</code>朝一<code>yi4</code>夕&quot;. Don't fully trust it.</p>

CC BY-SA 4.0
2020-10-19T15:24:03.037Z
Did you mean "第一次世界大战"? Also, to add, 一声 is yi1 when meaning the first tone in pronunciation, in other contexts it can be "counting" and pronounced yi4.
xngtng
CC BY-SA 4.0
2020-10-19T17:42:13.820Z
@zhantongz Yes you are right, I added the meaning there to reduce confusion. Depending on how you read it the meaning of these two words can change: "这一`yi2`次世界大战", and “叫了一`yi4`声” can be different from the meaning above.
River
CC BY-SA 4.0
2020-10-21T01:24:05.930Z
Are there any cases where `yi1` and `yi[24]` split differently than 二 and 两 or is it the same phenomenon?
Greg Nisbet
CC BY-SA 4.0
2020-10-21T02:03:13.063Z
@GregoryNisbet Here is an example 两百元 and 二百元 both work, but 两元 is correct while 二元 is weird. It is not related to the tone of the following character, because 二人同行 and 两人同行 both work. Whether 二 can be used for counting is very complex and I cannot conclude a good rule for now. But the rule for 一 is rather clearcut.
River
CC BY-SA 4.0
2020-10-21T04:01:57.563Z
Sorry, I was counting `yi2` and `yi4` as one thing in my follow-up question. Sometimes `yi1` is used. Sometimes `yi2/yi4-depending-on-the-next-tone` is used. I was wondering if the difference between `yi1` and `yi2/yi4-depending-on-the-next-tone` is the same as 二 vs 两. For example, when pronouncing `一百` are `yi1bai3` and `yi4bai3` both acceptable?
Greg Nisbet
CC BY-SA 4.0
2020-10-21T14:12:13.180Z
@GregoryNisbet only `yi4bai3` is correct but `yi1bai3` is not. This is different from 二 that 二百 and 两百 are both correct. That is why I say the rule for 一 is more clearcut than the rule for 二/两.
River
CC BY-SA 4.0
2020-10-21T14:19:32.463Z
Let me try to state it in a better way: If we only consider counting and indexing, `yi2/4` is always counting and `yi1` is always indexing. 两 is always counting, but 二 can be counting or indexing. However, when 二 can be used as counting is complex.
River
CC BY-SA 4.0
2020-10-19T02:20:17.130Z
2020-10-19T17:39:12.813Z
2020-10-19T17:39:12.813Z
River
<p><strong>The tone that 一 is pronounced with is sometimes hard to predict. Is there a good general rule for when to use yi1 and when to use yi2/yi4?</strong></p>
<hr />
<p>The 不 tone change rule is fairly straightforward and easy to remember. 不 is underlyingly <code>bu4</code> but changes to <code>bu2</code> before another fourth tone, within a phrase. Maybe the 不-change rule can be blocked by a phrase boundary; I am not sure.</p>
<p>The 一 tone change rule (YTC) is much more confusing. 一 can be pronounced as <code>yi1</code> or <code>yi2</code> or <code>yi4</code> according to both the surrounding environment and how 一 is being used.</p>
<p>For explaining the YTC, it almost seems like there are two separate words: one of which is always pronounced <code>yi1</code> and the other is pronounced as <code>yi4</code> or <code>yi2</code> using the same rules as 不.</p>
<p>Wiktionary lists a bunch of terms with 一 pronounced as <a href="https://en.wiktionary.org/wiki/Category:Mandarin_words_containing_%E4%B8%80_not_undergoing_tone_sandhi" rel="nofollow noreferrer">yi1</a>, as <a href="https://en.wiktionary.org/w/index.php?title=Category:Mandarin_words_containing_%E4%B8%80_undergoing_tone_sandhi_to_the_second_tone" rel="nofollow noreferrer">yi2</a> and as <a href="https://en.wiktionary.org/wiki/Category:Mandarin_words_containing_%E4%B8%80_undergoing_tone_sandhi_to_the_fourth_tone" rel="nofollow noreferrer">yi4</a>.</p>
<p>The <code>yi2</code> and <code>yi4</code> pronunciation appear to be in complementary distribution and never appear phrase-finally.</p>
<p><code>yi1</code> seems to appear in contexts where the corresponding two-related lexeme would use 二 instead of 两.</p>
<ul>
<li><a href="https://en.wiktionary.org/wiki/%E4%B8%80%E4%BE%86" rel="nofollow noreferrer">一来</a></li>
<li><a href="https://en.wiktionary.org/wiki/%E4%BA%8C%E4%BE%86" rel="nofollow noreferrer">二来</a></li>
</ul>
<p>I am not sure that the distribution of 两 versus 二 is the exactly the same as yi2/yi4 versus yi1. Is there a better rule of thumb?</p>

<grammar><sandhi>
General rule for when 一 is yi1 and when it's yi2/yi4
# General rule for when 一 is yi1 and when it's yi2/yi4

(Asked by: username_0 on 2020-10-19)

**The tone that 一 is pronounced with is sometimes hard to predict. Is there a good general rule for when to use yi1 and when to use yi2/yi4?**

---

The 不 tone change rule is fairly straightforward and easy to remember. 不 is underlyingly `bu4` but changes to `bu2` before another fourth tone, within a phrase. Maybe the 不-change rule can be blocked by a phrase boundary; I am not sure.

The 一 tone change rule (YTC) is much more confusing. 一 can be pronounced as `yi1` or `yi2` or `yi4` according to both the surrounding environment and how 一 is being used.

For explaining the YTC, it almost seems like there are two separate words: one of which is always pronounced `yi1` and the other is pronounced as `yi4` or `yi2` using the same rules as 不.

Wiktionary lists a bunch of terms with 一 pronounced as [yi1](https://en.wiktionary.org/wiki/Category:Mandarin_words_containing_%E4%B8%80_not_undergoing_tone_sandhi), as [yi2](https://en.wiktionary.org/w/index.php?title=Category:Mandarin_words_containing_%E4%B8%80_undergoing_tone_sandhi_to_the_second_tone) and as [yi4](https://en.wiktionary.org/wiki/Category:Mandarin_words_containing_%E4%B8%80_undergoing_tone_sandhi_to_the_fourth_tone).

The `yi2` and `yi4` pronunciation appear to be in complementary distribution and never appear phrase-finally.

`yi1` seems to appear in contexts where the corresponding two-related lexeme would use 二 instead of 两.

* [一来](https://en.wiktionary.org/wiki/%E4%B8%80%E4%BE%86)
* [二来](https://en.wiktionary.org/wiki/%E4%BA%8C%E4%BE%86)

I am not sure that the distribution of 两 versus 二 is the exactly the same as yi2/yi4 versus yi1. Is there a better rule of thumb?

**Tags:** grammar, sandhi


---

## 2 answers

### Answer by username_2 on 2020-10-19. Score: 4 | Accepted answer

The rule of thumb is the following.

When indexing it is `yi1`. For example

* 一楼 (first floor)
* 一声 (first tone)
* 第一 (the first)
* 一次世界大战 (World War I)
* 一来 (firstly)

When counting （the same rule as 不）

* it is `yi4` when followed by 1, 2, 3 tone
  + 一呼百应
  + 一毛不拔
  + 一百
  + 一扫而光
* it is `yi2` when followed by 4 tone
  + 一蹴而就
  + 一气呵成

For others, mostly `yi1`. For example

* 统一
* 均一
* 敬一丹（name）

As a comparison, 二 can be indexing or counting, but 两 is only counting.

PS: Most of the words in the Wikipedia list are right, but there are clear errors. For example "一`yi4`夫一`yi4`妻" appeared in 2 lists. And, it should be "一`yi4`朝一`yi4`夕". Don't fully trust it.

> **Comments:**
> **username_3**: Did you mean "第一次世界大战"? Also, to add, 一声 is yi1 when meaning the first tone in pronunciation, in other contexts it can be "counting" and pronounced yi4.  
> **username_2**: @zhantongz Yes you are right, I added the meaning there to reduce confusion. Depending on how you read it the meaning of these two words can change: "这一`yi2`次世界大战", and “叫了一`yi4`声” can be different from the meaning above.  
> **username_0**: Are there any cases where `yi1` and `yi[24]` split differently than 二 and 两 or is it the same phenomenon?  
> **username_2**: @GregoryNisbet Here is an example 两百元 and 二百元 both work, but 两元 is correct while 二元 is weird. It is not related to the tone of the following character, because 二人同行 and 两人同行 both work. Whether 二 can be used for counting is very complex and I cannot conclude a good rule for now. But the rule for 一 is rather clearcut.  
> **username_0**: Sorry, I was counting `yi2` and `yi4` as one thing in my follow-up question. Sometimes `yi1` is used. Sometimes `yi2/yi4-depending-on-the-next-tone` is used. I was wondering if the difference between `yi1` and `yi2/yi4-depending-on-the-next-tone` is the same as 二 vs 两. For example, when pronouncing `一百` are `yi1bai3` and `yi4bai3` both acceptable?  
> **username_2**: @GregoryNisbet only `yi4bai3` is correct but `yi1bai3` is not. This is different from 二 that 二百 and 两百 are both correct. That is why I say the rule for 一 is more clearcut than the rule for 二/两.  
> **username_2**: Let me try to state it in a better way: If we only consider counting and indexing, `yi2/4` is always counting and `yi1` is always indexing. 两 is always counting, but 二 can be counting or indexing. However, when 二 can be used as counting is complex.  

---

### Answer by username_1 on 2020-10-19. Score: 3

真的是个好问题，我原来总结过一些规律，但可能不靠谱。我感觉是根前后字（组合成的那个字）有关。
比如：

> 一二三四，读1声
>
> 一骑绝尘，读2声 （如果骑读作ji4的话）
>
> 一马当先，读4声

我发现，除了"一二三四"，如果"一"后面跟4声的词，"一"应该读2声，除了上面的例子还有诸如：

> 一箭双雕，一泄千里，一世英明……

如果"一"后面是1声、2声、或3声，"一"应该读4声，诸如：

> 一帆风顺、一尘不染、一鼓作气、一往无前……

也不知道准不准，欢迎提出反例。

> **Comments:**
> **username_2**: Older versions of the dictionary will say 一骑`yi2ji4`, but newer versions say `yi4qi2`, in PRC.  
> **username_1**: @username_2，确实，如果读骑读ji4，一应该读2声，如果骑读作qi2，一应该读4声。  

---

