---
title: About stemming
slug: '1576'
canonical_url: https://docs.coveo.com/en/1576/
collection: searching-with-coveo
source_format: adoc
---
# About stemming
[stemming](https://docs.coveo.com/en/3436/) is a process which reduces words to their stem, base, or root form.
The [index](https://docs.coveo.com/en/204/) uses the stem of each queried term to expand the [query](https://docs.coveo.com/en/231/) by searching for the original term and also indexed terms that share the same root.
This important automatic query expansion process often returns relevant results that wouldn't appear otherwise.

> **Note**
>
> Stemming only applies to words of three characters or more.

**Examples**

* Searching for a term in its singular form returns items that include both its singular and plural forms.
* The words _search_, _searching_, and _searched_ share the same root or stem: _search_.
When you query `searching`, the index returns items containing the words _searching_, _search_, _searches_, and _searched_.

## Stem expansions

For your index to use a certain term as a stem expansion, that term must appear in at least one indexed [item](https://docs.coveo.com/en/210/), where:

* The [search interface](https://docs.coveo.com/en/2741/) language matches one of the languages of that item.
* The term appears in the body of that item.

### Languages

In addition to only using terms from indexed items in the same language as the search interface, Coveo employs language-specific stemming algorithms in order to improve the relevance of stem expansions.
For more information, see a list of the [languages supported by Coveo](https://docs.coveo.com/en/1956/).



**Example**

The term _attention_ can stem to _attentio_ in English and _attenti_ in French.

The stem expansion _attentif_ is only relevant in French.


By default, the index assumes the search interface language to be the main language detected in indexed items.
However, your index may have multiple main languages.

Main languages are those that make up 90% of the indexed items, either individually or in combination with other languages.
The remaining 10% of indexed items are ignored during stemming.

> **Example**
>
> Your index contains 75% English documents, 15% French documents, and 10% Spanish documents.
> Both English and French are recognized as main languages.

You can, however, specify your preferred language in the Search API [`locale` query parameter](https://docs.coveo.com/en/13#operation/searchUsingPost-locale), and set the [`forwardLanguageToCoveoIndex` query parameter](https://docs.coveo.com/en/1491#forwardlanguagetocoveoindex-boolean) to `true`.
If the `forwardLanguageToCoveoIndex` isn't set to `true`, the index ignores the `locale` value, disregarding your preferred language.

### Stemming in field queries

You can also leverage stemming in field queries by enabling the [`stemming` option](https://docs.coveo.com/en/1833#stemming) of the target [fields](https://docs.coveo.com/en/200/).
That being said, keep in mind that doing so can impede performance.

## Ranking

The index gives higher [result ranking](https://docs.coveo.com/en/1624/) to items containing the original form of queried terms.
Moreover, the index calculates a correlation factor in your index between the searched term and every possible expansion.
In search results, highly correlated expansions are ranked higher than poorly correlated ones.
This decreases the risks of stemming confusion that could occur when words of different natures share the same stem.

**Example**

In English, the terms _university_ and _universe_ stem to the same root, although they're not semantically related.

When you search for `universe`, the Coveo index expands your query using terms from the _univer_ stem classes that can include _university._ However, since the terms _universe_ and _university_ rarely co-occur in your indexed items, items containing _university_ rank lower.

## Disabling stemming

While expanded queries are generally useful, you can disable the [stemming](https://docs.coveo.com/en/3436/) expansion when you want to [search for an exact term](https://docs.coveo.com/en/1815/) or [an exact phrase](https://docs.coveo.com/en/1686/).