From rhymes to networks (A new blog series in six steps)

Whenever one feels stuck in solving a particular problem, it is useful to split this problem into parts, in order to identify exactly where the problems are. The problem that is vexing me at the moment is how to construct a network of rhymes from a set of annotated poems, either by one and the same author, or by many authors who wrote during the same epoch in a certain country using a certain language.

For me, a rhyme network is a network in which words (or parts of words) occur as nodes, and weighted links between the nodes indicate how often the linked words have been found to rhyme in a given corpus

An example

As an example, the following figure illustrates this idea for the case of two Chinese poems, where the rhyme words represented by Chinese characters are linked to form a network (taken from List 2016).

Figure 1: Constructing a network of rhymes in Chinese poetry (List 2016)

One may think that it is silly to make a network from rhymes. However, experiments on Chinese rhyme networks (of which I have reported in the past) have proven to be quite interesting, specifically because they almost always show one large connected component. I find this fascinating, since I would have expected that we would see multiple connected components, representing very distinct rhymes.

It is obvious that some writers don't have a good feeling for rhymes and fail royally when they try to do it — this happens across all languages and cultures in which rhyming plays a role. However, it was much less obvious to me that rhyming can be seen to form at least some kind of a continuum, as you can see from the rhyme networks that we have constructed from Chinese poetry (again) in the past (taken from List et al. 2017).

Figure 2: A complete rhyme network of poems in the Book of Odes (ca. 1000 BC, List et al. 2017)

The current problem

My problem now is that I do not know how to do the same for rhyme collections in other languages. During recent months, I have thought a lot about the problem of constructing rhyme networks for languages such as English or German. However, I always came to a point where I feel stuck, where I realized that I actually did not know at all how to deal with this.

I thought, first, that I could write one blog post listing the problems; but the more I thought about it, I realized that there were so many problems that I could barely do it in one blogpost. So, I decided then that I could just do another series of blog posts (after the nice experience from the series on open problems in computational historical linguistics I posted last year), but this time devoted solely to the question of how one can get from rhymes into networks.

So for the next six months, I will discuss the four major issues that keep me from presenting German or English rhyme networks here and now. I hope that at the end of this discussion I may even have solved the problem, so that I will then be able to present a first rhyme network of Goethe, Shakespeare, or Bob Dylan. (I would not do Eminem, as the rhymes are quite complex, and tedious to annotate).

Summary of the series

Before we can start to think about the modeling of rhyme patterns in rhymed verse, we need to think about the problem in general, and discuss how rhyming shows up in different languages. So, I will start the series with the problem of rhyming in general, by discussing how languages rhyme, where these practices differ, and what we can learn from these differences. Having looked into this, we can think about ways of annotating rhymes in texts in order to acquire a first corpus of examples. So, the following post will deal with the problems that we encounter when trying to annotate the rhyme words that we identify in poetry collections.

If one knows how to annotate something, one will sooner or later get impatient, and long for faster ways to do these boring tasks. Since this also holds for the manual annotation of rhyme collections (which we need for our rhyme networks), it is obvious to think about automated ways of finding rhymes in corpora — that is, to think about the inference of rhyme patterns, which can also be done semi-automatically, of course. So the major problems related to automated rhyme detection will be discussed in a separate post.

Once this is worked out, and one has a reasonably large corpus of rhyme patterns, one wants to analyze it — and the way I want to analyze annotated rhyme corpora is with the help of network models. But, as I mentioned before, I realized that I was stuck when I started to think about rhyme networks of German and English (which are relatively easy languages, one should think). So, it will be important to discuss clearly what seems to be the best way to construct rhyme networks as a first step of analysis. This will therefore be dealt with in a separate blogpost. In a final post, I then plan to tackle the second analysis step, by discussing very briefly what one can do with rhyme networks.

All in all, this makes for six posts (including this one); so we will be busy for the next six months, thinking about rhymes and poetry, which is probably not the worst thing one can do. I hope, but I cannot promise at this point, that this gives me enough time to stick to my ambitious annotation goals, and then present you with a real rhyme network of some poetry collection, other than the Chinese ones I already published in the past.

References

List, Johann-Mattis, Pathmanathan, Jananan Sylvestre, Hill, Nathan W., Bapteste, Eric, Lopez, Philippe (2017) Vowel purity and rhyme evidence in Old Chinese reconstruction. Lingua Sinica 3.1: 1-17.

List, Johann-Mattis (2016) Using network models to analyze Old Chinese rhyme data. Bulletin of Chinese Linguistics 9.2: 218-241.

Networks in Chinese poetry

Structure in Poetry

Dealing with poetry is a dangerous topic in science, since we never know whether the structures we propose are really there or not. Once it comes to the search of structure in poetry, Matthew and Luke were right, since the ones who search will find, provided they have enough creativity.

When I had Latin lessons in school, some of my classmates were incredibly diligent in trying to find alliterations (instances in which words in a sentence start with the same letter) in Cicero's speeches. This was less out of interest in the structure of the speeches, but more an attempt to divert the teacher's attention away from translation.

The problem with structure in poetry is that we never know in the end whether the people who created the poetry did things with purpose or not. Consider, for example, the following lines of a famous verse:

Apart from the fact that people might disagree whether songs by Eminem are poetry, it is interesting to look at the structures one may (or may not) detect. We know that rap and hip hop allow for rather loose rhyming schemes, which may give the impression that they were produced in an ad-hoc manner. We know also that the question of what counts as a rhyme is strictly cultural. In German, for example, employ could rhyme with supply (thanks to Goethe and other poets who would superimpose to the standard language rhyme patterns that made sense in their home dialect). If I was given Eminem's poem in an exam, I would mark its rhyming structure as follows:

I do not know whether any teacher of English would agree that music can rhyme with own it, but if Germans can rhyme [ai] (as in supply) with [ɔi] (as in employ), why not allow [ɪk] (as in music) to rhyme with [ɪt] (as in own it)? I bet that if one made an investigation of all rhymes that Bob Dylan has produced so far, we would find at least a few instances where he would tolerate Eminem's rhyme pattern.

The point here is that rhymes are important evidence to infer how Ancient Chinese was pronounced.

The Pronunciation of Ancient Chinese

The Chinese writing system gives only minimal hints regarding the pronunciation of the characters. If one writes a character like 日 which means 'sun', the writing system gives us no clue as to its pronunciation; and from the modern form in which the character is written, it is also difficult to see the image of a sun in the character. Thus, the current situation in Chinese linguistics is that we have very ancient texts, dating at times back to 1000 BC, but we do not have a real clue as to how the language was pronounced by then.

That it was pronounced differently is clear from — ancient Chinese poetry. When reading ancient poems with modern pronunciations, one often finds rhyme patterns which do not sound nice. Consider the poem from Ode 28 of the Book of Odes (Shījīng 詩經), an ancient collection of poems written between 1050 and 600 BC (translation from Karlgren 1950):

Here, we find modern rhymes between fēi and guī which is fine, since the transliteration fails to give the real pronunciation, which is [fəi] versus [kuəi]; but we also find [in] rhyming with [nan], which is so strange (due to the strong difference in the vowels) that even Bob Dylan and Eminem probably would not tolerate it. But if we do not tolerate this rhyming pattern, and if we do not want to assume that the ancient masters of Chinese poetry would simply fail in rhyming, we need to search for some explanation as to why the words do not rhyme. The explanation is, of course, language evolution — The sound systems of languages constantly change, and if things do not rhyme with our modern pronunciation, they may have been perfect rhymes when they were originally created.

When Chinese scholars of the 16th century, who investigated their ancient poetry, became aware of this, they realized that the poetry could be a clue to reconstruct the ancient pronunciation of their language. Then they began to investigate the ancient poems of the Book of Odes systematically for their rhyme patterns. It is thanks to this work on early linguistic reconstruction by Chinese scholars, that we now have a rather clear picture of how Ancient Chinese was pronounced (see especially Baxter 1992, Sagart 1999, and Baxter and Sagart 2014).

Networks in Chinese Rhyme Patterns

But where are the networks in Chinese poetry, which I promised in the title of this post? They are in the rhyme patterns — It is rather straightforward to model rhyme patterns in poetry with the help of networks. Every node is a distinct word that rhymes in at least one poem with another word. Links between nodes are created whenever one word rhymes with another word in a given stanza of a poem. So, even if we take only two stanzas of two poems of the Book of Odes, we can already create a small network of rhyme transitions, as illustrated in the following figure:

One needs, of course, to be careful when modeling this kind of data, since specific kinds of normalizations are needed to avoid exaggerating the weight assigned to specific rhyme connections. It is possible that poets just used a certain rhyme pattern because they found it somewhere else. It is also not yet entirely clear to me how to best normalize those cases in which more than two words rhyme with each other in the same stanza.

But apart from these rather technical questions, it is quite interesting to look at the patterns that evolve from collecting rhyme patterns of all poems found in the Book of Odes, and plotting them in a network. I prepared such a dataset, using the rhyme assessments by Baxter (1992). The whole data set is now available in the form of an interactive web-application at http://digling.org/shijing.

In this application, one can browse all characters that appear in potential rhyme positions in all 305 poems that constitute the Book of Odes. Additional meta-data, like reconstructions for the old pronunciations following Baxter and Sagart (2014), which were kindly provided by L. Sagart, have also been added. The core of the app is the "Poem View", by which one can see a poem, along with reconstructions for the rhyme words, and an explicit account of what experts think rhymed in the classical period, and what they think did not rhyme. The following image gives a screanshot of the second poem of the Book of Odes:

But let's now have a look at the big picture of the network we get when taking all words that rhyme into account. The following image was created with Cytoscape:

As we can see, the rhyme words in the 305 poems almost constitute a small world network, and we have a very large connected component. For me, this was quite surprising, since I was assuming that rhyme patterns would be more distinct. It would be very interesting to see a network of the works of Shakespeare or Goethe, and to compare the amount of connectivity.

There are, of course, many things we can do to analyze this network of Chinese poetry, and I am currently trying to find out to what degree this may contribute to the reconstruction of the pronunciation of Ancient Chinese. But since this work is all in a preliminary stage, I will restrict this post by showing how the big network looks if we color the nodes in six different colors, based on which of the six main vowels ([a, e, i, o, u, ə]) scholars usually reconstruct in the rhyme word for Ancient Chinese:

As can be seen, even this simple annotation shows how interesting structures emerge, and how we see more than before.

Many more things can be done with this kind of data. This is for sure. We could compare the rhyme networks of different poets, maybe even the networks of one and the same poet at different stages of their life, asking questions like: "do people rhyme more sloppy, the older they get?" It's a pity that we don't have the data for this, since we lack automatic approaches to detect rhyme words in text, and there are no manual annotations of poem collections apart from the Book of Odes that I know of.

But maybe, one day, we can use networks to study the dynamics underlying the evolution of literature. We could trace the emergence of rap and hip hop, or the impact of the "Judas!"-call on Dylan's rhyme patterns, or the loss of structure in modern poetry. But that's music from the future, of course.

References

• Baxter, William H. (1992) A handbook of Old Chinese phonology. Berlin: De Gruyter.
• Baxter, William H. and Sagart, Laurent (2014) Old Chinese. A new reconstruction. Oxford: Oxford University Press.
• Karlren, Bernhard (1950) The Book of Odes. Stockholm: Museum of Far Eastern Antiquities.
• Sagart, Laurent (1999) The roots of Old Chinese. Amsterdam: John Benjamins.