# How does SpaCy make its dependency tree?

I discovered that SpaCy had the ability to make dependency trees. For instance given a question “To whom did the Virgin Mary allegedly appear in 1858 in Lourdes France?”

We can create its dependency tree with the following code :

I know that it is a set of edges $E$ which represents the relations of labeled dependencies of the particular analysis of $G$. More precisely, an edges $w_i, r, w_j \in E$ represents a relationship of dependence of head $w_i$ towards $w_j$ labeled with the type of relationship. A dependency graph $G$ is therefore a set of dependency relations between tokens in $S$.

For instance we present the dependency graph that follows by the following graph:

• $G = (V, E)$
• $V = \{\mbox{It, is, a, replica, of, the, grotto, at, Lourdes, France, where, the, Virgin, Mary}\\ \mbox{reputedly, appeared, to, Saint, Bernadette ...}\}$
• $E=\{\mbox{(is, NSUBJ, It), (is, ATTR, a replica), (a replica, PREP, of),...}\}$

But I was wondering how does $E$ was made with Spacy? How does the dependency tree method chose which node is the root and which aren't?

I have heard from this French paper that, on the one hand that $E \subseteq V \times R \times V$ where $R = {r_1, ..., r_m}$ is a finite set of dependency possible names that we call dependency relationship. They can link two tokens in a sentence. A dependency name $r \in R$ is also called a labeled edge. For example, a relationship between a head verb and its subject can be noted with $r = SBJ$.

On the other hand it was an analysis problem of finding the dependency graph that has the highest score for an input sentence.

Yet I haven't seen where there is a child-children ordering creation.

The context is that I am using SpaCy dependency trees to get the root of sentences of a paragraph and match it with the root of a question.

For the question given above we have to find the answer in the following paragraph :

Context — “Architecturally, the school has a Catholic character. Atop the Main Building\’s gold dome is a golden statue of the Virgin Mary. Immediately in front of the Main Building and facing it, is a copper statue of Christ with arms upraised with the legend “Venite Ad Me Omnes”. Next to the Main Building is the Basilica of the Sacred Heart. Immediately behind the basilica is the Grotto, a Marian place of prayer and reflection. It is a replica of the grotto at Lourdes, France where the Virgin Mary reputedly appeared to Saint Bernadette Soubirous in 1858. At the end of the main drive (and in a direct line that connects through 3 statues and the Gold Dome), is a simple, modern stone statue of Mary.”

The expected answer is  “Saint Bernadette Soubirous”. The sentence having the answer — is “It is a replica of the grotto at Lourdes, France where the Virgin Mary reputedly appeared to Saint Bernadette Soubirous in 1858”. Its dependency tree of the sentence containing the answer is :