Likewise, words with the same literal meaning in either dialect may differ in register. éryǐ 'that's all, only' is common both in spoken and written Guoyu, influenced by speech patterns in Minnanyu, but in Putonghua the word is largely confined to formal, written contexts. Preference for the expression of modality often differs among northern Mandarin speakers and Taiwanese, as evidenced by the selection of modal verbs. For example, Taiwanese Mandarin users strongly prefer yào and búyào over děi and bié, respectively, to express 'must' and 'must not', compared to native speakers from Beijing. However, yào and búyào are also predominantly used among Mandarin speakers from the south of the mainland. Both pairs are grammatically correct in either dialect. Words specific to Guoyu Some words in Putonghua may not exist in Guoyu and vice versa. Authors of the Dictionary of Words Which Differ Across the Taiwan Strait () estimate there are about 2,000 words unique to Guoyu, around 10% of which come from Minnanyu. Additionally, many terms unique to Guoyu were adopted from Japanese as a result of Taiwan's status as a Japanese colony during the first half of the 20th century. Some of vocabulary differences stem from different social and political conditions, which gave rise to concepts that were not shared between the mainland and Taiwan, e.g. fúcǎi, a common abbreviation for the China Welfare Lottery of the People's Republic of China, or shíbāpā, which refers to the 18% preferential interest rate on civil servants' pension funds in Taiwan. ( pā as "percent" is also unique to Guoyu.) Particles Modal particles convey modality, which can be understood as a speaker's attitude towards a given utterance (e.g. of necessity, possibility, or likelihood that the utterance is true). Modal particles are common in Chinese languages and generally occur at the end of sentences and so are commonly called sentence-final particles or utterance-final particles.Guoyu employs some modal particles that are rare in Putonghua. Some are entirely unique to spoken, colloquial Taiwan Guoyu, and identical particles may also have different meanings in Putonghua and Guoyu. Conversely, particles that are common in Putonghua — particularly northern Putonghua, such as that spoken in Beijing — are very rare in Guoyu. Examples include () bei, me, and () bàle. lā is a very common modal particle in Guoyu, which also appears in Putonghua with less frequency and always as a contraction of le and a. In Guoyu,