In Spanish, all nouns are either masculine or feminine in gender. This is merely a grammatical concept; it does not mean that Spanish speakers perceive things as having male or female attributes. Nouns that refer to male people or animals, and most nouns that end in -o are masculine in gender. (One common exception is la mano, æthe handÆ). Most nouns that refer to female people or animals, and most nouns that end in -a, -ción, -dad or -tad are feminine in gender. (Some exceptions are el día, el problema, el drama, and el programa). Nouns that have other endings, or nouns that are exceptions to the rules, must be memorized: el billete, el dólar, el valor, etc.