Is Cantonese a better choice in looking for characters in rime dictionary (韻書)?
Though I emphasize Cantonese here, any southern dialect / language that still have entering tone (入聲); e.G. Hakka and Min, would have a similar scenario.

The oldest rime dictionary (韻書) I found, is 鉅宋廣韻, printed in 1169; that the national archives of Japan has a downloadable copy:

https://www.Digital.Archives.Go.Jp/das/pickup/view/category/categoryarchives/0500000000/0511000000/00

To look for a character in rime dictionary, you just need to know how to pronounce it, in Cantonese.

Let's try to find 入 (u+5165)


I know it's pronounced as sound file; which is straight &amp; short (直而促). A clear, significant sign that 入 (u+5165) is in entering tone (入聲).
Meanwhile, if I try to write it's romanisation( I used sydney lay scheme, you may use others), it would be "yap". It ends with -p, another significant sign that 入 (u+5165) is in entering tone (入聲).
Pick up volume 5 of 鉅宋廣韻 {volume I &amp; II is for level tone (平聲), volume III for rising tone (上聲), volume IV for departing tone (去聲); and volume V for entering tone (入聲)}.


https://www.Digital.Archives.Go.Jp/das/pickup/view/detail/detailarchives/0511000000/0000000873/00


Turn to the index page of volume 5, read from right to left, top to bottom; try to find any character that has a similar pronunciation with "入".





Voilà, 緝第二十六; the character 緝 has a similar pronunciation, and, the "七入" above 緝 is used for 反切, which has "入" :)
Lastly, turn to 緝第二十六. Well, no page number in ancient Chinese books. You guess roughly, and turn the page :( read again from right to left, then top to bottom. Voilà, it's there:




Ok, 緝第二十六 is page 34, 入 is page 35:

It was written: 入﹒得也﹒ 内也﹒納也﹒人執切

By the way, 内 (u+5185) is not 內 (u+5167); anyone see the difference?

Now, this comment is quoted from the above mentioned thread:


  Point 3 is straight factually incorrect that people can use 韻書 (rime dictionaries) with Cantonese but not Mandarin. It is just factually wrong and unlike point 1, there even is no dispute about this. Rime dictionaries do not apply as much to Cantonese as they do not to Mandarin. – rethliopuks


Now, may I ask, how can anyone searches for the character 入 (u+5165), using Mandarin?

According to Mandarin, 入 is pronounced as rù (去聲). So, anyone picking volume IV of 鉅宋廣韻 would be trapped, it's not in this volume.

I used 入 as the only example to show the uselessness of Mandarin in looking for characters in rime dictionaries. If anyone have time and effort, and wants to verify this context, try two more entering tone (入聲) characters:

哲 (u+54f2)

福 (u+798f)

Have fun　

This answer is already quite long, I would say that most, or, all entering tone (入聲) characters would have the same scenario.

Last, I would conclude that, to look for character in rime dictionaries (韻書), Cantonese, or other dialects / languages that have entering tone (入聲); are a better choice than Mandarin.

Or, a direct, maybe provoking statement would be:

Since Mandarin lost the entering tone (入聲), it's useless in looking up characters in rime dictionaries (韻書).

Stay tuned, there'll be another thread about entering tone (入聲) and old Chinese; Cantonese is again better.