Which language matters? Number comparison in bilingualsNumbers are ubiquitous in modern life. We encounter exact numbers in different formats, most frequently as spoken or written number words (‘twenty-four’) or Arabic digits (24). While number words are language-specific1, Arabic digits are often seen as an abstract, pure and non-linguistic number format. However, even basic number processing of Arabic digits is influenced by the language background of the person processing them2,3.Strong support for an influence of language background on number processing comes from developmental research4,5,6,7,8,9. For example, whether a language has a clear singular/plural distinction (one frog, two frogs, three frogs, four frogs) might influence the acquisition of the meaning of the number word ‘one’10. English-speaking children typically learn the meaning of ‘one’ around 24 to 26 months10,11. But in languages with less consistent or without obligatory associations of number words with singular/plural nouns, such as Japanese, children learn the meaning of the number word ‘one’ several months later10,12,13 than in English.Moving on to larger number words, number word construction is entirely regular in some languages, such as Japanese and Mandarin. Once children have learnt the number words one to ten in these languages, they can combine these ten elements to create all number words up to 99. Several studies investigated the acquisition of teen numbers (11–19) in different languages and reported a delay in number word acquisition for numbers larger than ten for languages for which number words are less regular14,15,16,17.One’s language acquired from birth does not only influence number word acquisition and numerical and arithmetic development4,18,19,20,21,22, it also affects number processing of Arabic digits in adults2,22,23,24. One effect that has often been used to investigate language influences on number processing in adults is the unit-decade compatibility effect25. This effect is observed in multi-digit number comparison, when participants are asked to choose, for example, the numerically larger number out of two-digit number pairs in Arabic format. A number pair is considered unit-decade compatible when comparing tens and units leads to similar response biases (e.g., 42_57: 4