# Are crossing dependencies really scarce?

## TL;DR

This paper investigates the scarcity of crossing dependencies in real sentences by comparing actual sentence structures to various baseline models, concluding that crossings are genuinely rare despite the trees' hubiness.

## Contribution

It provides a quantitative analysis showing that crossing dependencies are scarce in real sentences and challenges the assumption that this scarcity is due to the hubiness of syntactic trees.

## Key findings

- Crossings are significantly fewer in real sentences than in random models.
- Real sentences are close to linear trees despite the potential for many crossings.
- The scarcity of crossings is not explained by the hubiness of syntactic trees.

## Abstract

The syntactic structure of a sentence can be modelled as a tree, where vertices correspond to words and edges indicate syntactic dependencies. It has been claimed recurrently that the number of edge crossings in real sentences is small. However, a baseline or null hypothesis has been lacking. Here we quantify the amount of crossings of real sentences and compare it to the predictions of a series of baselines. We conclude that crossings are really scarce in real sentences. Their scarcity is unexpected by the hubiness of the trees. Indeed, real sentences are close to linear trees, where the potential number of crossings is maximized.