System of conventional spoken or written symbols used by people in a shared culture to communicate with each other. A language both reflects and affects a culture's way of thinking, and changes in a culture influence the development of its language. Related languages become more differentiated when their speakers are isolated from each other. When speech communities come into contact (e.g., through trade or conquest), their languages influence each other. Most existing languages are grouped with other languages descended genetically from a common ancestral language (see historical linguistics). The broadest grouping of languages is the language family. For example, all the Romance languages are derived from Latin, which in turn belongs to the Italic branch of the Indo-European language family, descended from the ancient parent language, Proto-Indo-European. Other major families include, in Asia, Sino-Tibetan, Austronesian, Dravidian, Altaic, and Austroasiatic; in Africa, Niger-Congo, Afro-Asiatic, and Nilo-Saharan; and in the Americas, Uto-Aztecan, Maya, Otomanguean, and Tupian. Relationships between languages are traced by comparing grammar and syntax and especially by looking for cognates (related words) in different languages. Language has a complex structure that can be analyzed and systematically presented (see linguistics). All languages begin as speech, and many go on to develop writing systems. All can employ different sentence structures to convey mood. They use their resources differently for this but seem to be equally flexible structurally. The principal resources are word order, word form, syntactic structure, and, in speech, intonation. Different languages keep indicators of number, person, gender, tense, mood, and other categories separate from the root word or attach them to it. The innate human capacity to learn language fades with age, and languages learned after about age 10 are usually not spoken as well as those learned earlier. See also dialect.