L1 is a speaker's first language. L2 is the second, L3 the third etc.

Two students talking at a desk

L1 is also sometimes referred to as "mother tongue".

Example

A learner whose L1 is Spanish may find Portuguese and Italian easy languages to learn because of a fairly close connection between the languages.

In the classroom
L1 interference - where a speaker uses language forms and structures from their first language in language they are learning - is an area many teachers are concerned with. In a mono-lingual class where the teacher also speaks the L1, it is easier to identify interference and address it, often discussing it explicitly with learners.

Further links:

https://www.teachingenglish.org.uk/article/mother-tongue-other-tongue

https://www.teachingenglish.org.uk/article/translation-activities-language-classroom

https://www.teachingenglish.org.uk/article/planning-a-writing-lesson

https://www.teachingenglish.org.uk/article/how-should-learners-own-languages-be-used-classroom

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