[[similarity]] === `similarity` Elasticsearch allows you to configure a scoring algorithm or _similarity_ per field. The `similarity` setting provides a simple way of choosing a similarity algorithm other than the default TF/IDF, such as `BM25`. Similarities are mostly useful for <> fields, but can also apply to other field types. Custom similarities can be configured by tuning the parameters of the built-in similarities. For more details about this expert options, see the <>. The only similarities which can be used out of the box, without any further configuration are: `classic`:: The Default TF/IDF algorithm used by Elasticsearch and Lucene. See {defguide}/practical-scoring-function.html[Lucene’s Practical Scoring Function] for more information. `BM25`:: The Okapi BM25 algorithm. See {defguide}/pluggable-similarites.html[Pluggable Similarity Algorithms] for more information. The `similarity` can be set on the field level when a field is first created, as follows: [source,js] -------------------------------------------------- PUT my_index { "mappings": { "my_type": { "properties": { "default_field": { <1> "type": "text" }, "bm25_field": { "type": "text", "similarity": "BM25" <2> } } } } } -------------------------------------------------- // CONSOLE <1> The `default_field` uses the `classic` similarity (ie TF/IDF). <2> The `bm25_field` uses the `BM25` similarity.