The evolution of relative clauses in the I ce P a HC treebank (2025)

Abstract: We examine how the elements that introduce relative clauses, namely relative complementizers and relative pronouns, evolve over the history of Icelandic using the phrase structure analysis of the IcePaHC treebank. The rate of these elements changes over time and, in the case of relative pronouns, is subject to effects of genre and the type of gap in the relative clause in question. Our paper is a digital humanities study of historical linguistics which would not be possible without a parsed corpus that spans all centuries involved in the change. We relate our findings to studies on the Constant Rate Effect by analyzing these effects in detail.

## introduction in this paper, we report on a study that analyzes the historical evolution of the elements that introduce relative clauses in icelandic. two types of elements are involved. first, we have relative complementizers, the elements sem and er, similar to english that. second, we have relative pronouns, words that start with hv-, similar to english wh-words. examples of the relative complementizers are given in (1) and (2) and a relative pronoun is shown in (3). historically, the relative complementizer used to be er, but over time, this element has been replaced with sem, which has the same syntactic distribution and function. these elements are very common in historical texts, er is dominant in the earliest texts and sem in modern texts. in contrast, relative pronouns like hverja 'whom' in example (3) have never been very common, but for a while they gained some popularity among writers who produced icelandic texts. the present study examines this historical development. as thousands of examples need to be analyzed in order to uncover the relevant facts, a parsed corpus (a treebank) is essential. that is a collection of texts that has been annotated in terms of syntactic structure. we use the icepahc treebank for this purpose @xcite . this means that different uses of the words sem and er are disambiguated. for example, sem can be a relative complementizer or a comparative complementizer and er can either be a relative complementizer or an inflected form of the verb 'to be' in icelandic. the paper is organized as follows: the background section introduces relative complementizers sem and er as historically competing forms and provides methodological details on the treebank. we extract all relevant examples of the environments in question and report on the findings drawn from these in the sections on relative complementizers and relative pronouns. we show that the change from er to sem in the history of icelandic follows a very regular s-shaped curve and discuss how changes in the introduction of relative clauses relate to the constant rate effect @xcite , an important property of syntactic change in the languages of the world. the main findings are then summarized in the conclusion. ## background while some traditional texts categorize the icelandic words sem and er as relative pronouns, @xcite argues that they are in fact relative complementizers. this is because they do not pattern with pronouns in their formal properties. unlike icelandic pronouns, they do not manifest case declension and they cannot be the complements of prepositions. they also always appear at the beginning of subordinate clauses. this is in contrast with actual relative pronouns, which also appear in icelandic, that start with hv, such as hver 'who', much like english wh-words. of these elements that introduce relative clauses, the complementizers are much more common. there are some more examples of elements introducing relative clauses but the other types are comparatively rare. these include clauses that begin with the form sá, typically used as demonstrative, and it has also been noted that the icelandic author, halldór laxness, sometimes uses og, typically a coordinating conjunction 'and', to start his relative clauses @xcite . the older icelandic texts use er as a relative complementizer (sometimes written es). this is a frozen form of what used to be a pronoun historically @xcite . the form sem then evolves from the comparative particle sem, but sem as a relative complementizer is not attested in the oldest written sources, i.e., runes from before the year 1000 @xcite . the change from er to sem is quite interesting from the point of view of historical linguistics because the entire change is attested in the historical record, unlike some changes that as linguists we only get to observe once the change is underway. it has long been noted that when two linguistic forms compete for use, the transition from one to the other may follow an s-shaped curve if the rate of use is plotted against a time axis. this type of a historical change has been derived from certain hypotheses about how children acquire language @xcite . one well-known example of a syntactic change that follows an s-shaped curve is the rise of do-support in the history of english @xcite . in his analysis of do-support, kroch proposes a constant rate effect for historical change, such that when a change applies in more than one syntactic context, the rate of change is the same across contexts, even though the rate of use is different depending on context. we revisit s-curves and the constant rate effect below, as testing these hypotheses/ effects sheds light on the theoretical implications of our study. ## the icelandic parsed historical corpus the icelandic parsed historical corpus, icepahc @xcite @xcite , is a manually annotated phrase structure treebank, developed in the tradition of the penn parsed corpora of historical english (ppche) @xcite . while the penn treebank @xcite was the first major treebank to be developed and remains the best known such resource, various lessons were learned during its development and some of these led to changes in the annotation scheme for the historical corpora, notably including a more flat phrase structure for constructions where structural ambiguity makes consistent and informative annotation challenging. the icelandic treebank builds on this experience by adopting an annotation scheme which is in most respects identical to the ppche scheme, only adjusting it in minor ways where icelandic requires additional information. the modifications include more morphological information at the pos-tag level, such as the annotation of morphosyntactic case features. icepahc consists of one million words of text, all of which have been manually annotated. this includes samples from 61 texts and in the corpus distribution, a plain text version of each text, along with a version that is pos-tagged and lemmatized, and finally, and most importantly, a version that has been annotated for phrase structure according to the ppche guidelines. since this is a historical corpus, an even distribution of samples from all centuries is emphasized and the corpus contains texts from the 12th century to the 21st century inclusive. ## relative complementizers let us first consider the evolution of relative complementizer over time, i.e. how sem replaces er as the form used for this purpose in icelandic. figure 1 shows how these forms evolve based on our data from the icepahc corpus. first consider part (a) of the figure. this shows the empirical rate of sem for each century of written text, as a proportion of total sem + er clauses, split up by the type of subject gap in the clause. distinctions between types of subject gap are demonstrated by the examples in (4) and ( 5 ). these examples show that the empty slot in the relative clause can correspond to constituents that have a different grammatical status. this is interesting because previous research has found that subjects are more accessible in processing than objects and objects are more accessible than obliques @xcite . being accessible in this context means that for comprehension purposes, less accessible elements suffer from lower accuracy, longer processing time, and greater working memory burden. for production, less accessible objects result in slower responses, more errors and more omissions or substitutions. additionally, in both child and second language acquisition, they are characterized by later acquisition and greater avoidance. the first thing to notice about figure 1 (a ) is that the empirical rate of sem over time follows a very regular curve. this is interesting because even the well-known s-curve from @xcite that describes the rise of do-support in the history of english is quite wiggly. the only century that appears to deviate from a regular rise is the 19th century and it turns out that this exception has a straightforward explanation. the corpus contains two texts from the 19th century, sagan af heljarslóðarorrustu and hellismanna saga, both of which manifest a low rate of sem because they are intentionally written in an archaic style. these two texts contribute substantially to the overall rate for the 19th century. apart from this, the curve is remarkably regular. furthermore, if we look at figure 1 (b ), we see the predicted probabilities of sem over the same centuries, again split by gap type, and in this case based on the output of a mixed-effects regression model. the model is built with usage of sem as the response variable and the predictors century, gap type, as well as an interaction between century and gap type; text-id was added as random effect. all of these predictors are highly significant as shown in the icepahc corpus contains metadata about the text genre (e.g., narrative or religious text), as mentioned above. unexpectedly, genre was not significant in the model selection process for relative complementizers. this suggests that other factors such as century or gap type were better suited to explain the observed variation in the data set. we considered genre because religious texts might be expected to be more conservative than narratives; those are the two genres for which we have most data. however, such an effect is not found. ## relative pronouns as outlined, in addition to relative complementizers, another way to introduce relative clauses in icelandic is relative pronouns. figure 1 (c ) shows the rate of relative pronouns over time, as a proportion of all relative clauses, split up by the type of gap (subject, object, and other). it reveals a relatively low rate of relative pronoun use across all centuries considered here. the highest rates of relative pronouns can be found between the 16th and late 18th centuries, with a peak of about 20% in the 17th century. thus, relative complementizers, the alternative to relative pronouns, remain the most common form historically. this distribution also affects the type of gap. for subject gaps, relative complementizers are more readily available since they are the most common form, so they are chosen more frequently. inversely, relative pronouns are less likely to be used with subject gaps (see figure 1 (d) ; interaction the regression model for relative pronouns (response variable) includes century, type of gap, and genre as fixed effects, an interaction between century and type of gap, and finally text-id as random effect (see further analysis reveals that besides century and the type of gap, genre is also an important factor in conditioning relative pronoun use (see ## conclusion in this paper, we have shown that using the phrase structure analysis of the icepahc treebank provides valuable insights into the diachronic evolution of icelandic relative clauses. from the 12th century up until the 21st century, relative complementizers have been more common than relative pronouns. the choice of complementizer is conditioned by the type of gap in relation to frequency, e.g., sem is selected more frequently for subject gaps when sem is the most common form and er is selected more frequently for subject gaps when er is more common. for relative pronouns, the analysis reveals a genre effect, which is not present for relative complementizers. we find that the relative pronouns are used most often in biographies and religious texts in the 16th to 18th century and they are especially frequent in clauses whose gap is not an argument, i.e., not a subject or an object, but rather something else. in sum, we add new evidence to an ever-growing body of research on icelandic using language technology resources. the findings of this study further inform future work on the constant rate effect, providing another test case for this effect.