The use of accent marks in printing varies by period, due to reforms successively promulgated by the Spanish Royal Academy. For example, many of the words that are today standardly written with an accent mark appeared more often without it up until around 1880. These include words with final stress ending in "-n" (e.g. ', ', ', ', '—but not future-tense verb forms like ', '); verbs in the imperfect tense (e.g. ', '); the possessives "mío" and "mía"; and the word '. Meanwhile, one-letter words other than the conjunction "y"—namely the preposition "a" and the conjunctions "e" (the form of "y" before an sound), "o", and "u" (form of "o" before [o)—are generally written with accent marks from the mid-1700s to 1911. The accent-marked infinitives such as ', ', ' began to outnumber the unaccented form around 1920, dropped the accent mark again in 1952, and regained it in 1959. Monosyllabic preterit verb forms such as ' and "" were written with accent marks before the 1952.