Sanskrit prosody includes linear and non-linear systems. The system started off with seven major metres, according to Annette Wilke and Oliver Moebus, called the "seven birds" or "seven mouths of Brihaspati", and each had its own rhythm, movements and aesthetics wherein a non-linear structure (aperiodicity) was mapped into a four verse polymorphic linear sequence. A syllable in Sanskrit is classified as either "laghu" (light) or "guru" (heavy). This classification is based on a "matra" (literally, "count, measure, duration"), and typically a syllable that ends in a short vowel is a light syllable, while those that end in consonant, "anusvara" or "visarga" are heavy. The classical Sanskrit found in Hindu scriptures such as the "Bhagavad Gita" and many texts are so arranged that the light and heavy syllables in them follow a rhythm, though not necessarily a rhyme.