acquired the sense of futurity (cf. I have to sing). Finally it became true future tense in almost all Romance languages and the auxiliary became a full-fledged inflection (cf. Spanish , , , French , , , Italian , , , 'I will sing', 'you will sing', 's/he will sing'). In some verbs the process went further and produced irregular forms [cf. Spanish haré (instead of , 'I'll do') and tendré (not , 'I'll have', the loss of e followed by epenthesis of d is especially common)] and even regular forms (the change of the a in the stem to e in has affected the whole class of conjugation type I Italian verbs). Japanese compound verbs An illustrative example of this cline is in the orthography of Japanese compound verbs. Many Japanese words are formed by connecting two verbs, as in , and in Japanese orthography lexical items are generally written with kanji (here and ), while grammatical items are written with hiragana (as in the connecting ). Compound verbs are thus generally written with a kanji for each constituent verb, but some suffixes have become grammaticalized, and are written in hiragana, such as , from , as in . Historical linguistics In Grammaticalization (2003) Hopper and Traugott state that the cline of grammaticalization has both diachronic and synchronic implications. Diachronically (i.e. looking at changes over time), clines represent a natural path along which forms or words change over time. However, synchronically (i.e. looking at a single point in time), clines can be seen as an arrangement of forms along imaginary lines, with at one end a 'fuller' or lexical form and at the other a more 'reduced' or grammatical form. What Hopper and Traugott mean is that from a diachronic or historical point of view, changes of word forms is seen as a natural process, whereas synchronically, this process can be seen as inevitable instead of historical. The studying and documentation of recurrent clines enable linguists to form general laws of grammaticalization and language change in general. It plays an important role in the reconstruction of older states of a language. Moreover, the documenting of changes can help to reveal the lines along which a language is likely to develop in the future. Unidirectionality hypothesis The unidirectionality hypothesis is the idea that grammaticalization, the development of lexical elements into grammatical ones, or less grammatical into more grammatical, is the