The written accent is very important in speaking Spanish correctly. The written accent indicates where the stress on a word falls if that stress doesn't follow the conventional rules of Spanish pronunciation. The stress accent is always on a vowel with one exception. Sometimes the accent is needed to distinguish between nouns that are spelled identically, such as 'papa' (potato) and 'papá' (father). It can also be used to distinguish between past and present tenses of verbs, such as 'compro' (I buy) and 'compró' (he bought). Accents can also be used to indicate questioning, such as with the word 'que' which is unaccented in statements, but accented in questions. The exception to the rule above is the tilde. A tilde is a diacritical mark (an accent) that is placed over a letter to change its pronunciation. In Spanish, it is used over the letter 'n' to indicate an 'ny' sound, as in mañana. It is important to be able to pronounce this sound to speak Spanish correctly. The tilde is also used in other languages, such as Portuguese and Vietnamese, though it is used with different letters in those languages. Tildes can also be seen in some web addresses.