from Modest Mussorgsky's Pictures at an Exhibition (1874) is a good example. The opening measures are shown below: Igor Stravinsky's The Rite of Spring (1913) is famous for its "savage" rhythms. Five measures from "Sacrificial Dance" are shown below: In such cases, a convention that some composers follow (e.g., Olivier Messiaen, in his La Nativité du Seigneur and Quatuor pour la fin du temps) is to simply omit the time signature. Charles Ives's Concord Sonata has measure bars for select passages, but the majority of the work is unbarred. Some pieces have no time signature, as there is no discernible meter. This is sometimes known as free time. Sometimes one is provided (usually ) so that the performer finds the piece easier to read, and simply has "free time" written as a direction. Sometimes the word FREE is written downwards on the staff to indicate the piece is in free time. Erik Satie wrote many compositions that are ostensibly in free time but actually follow an unstated and unchanging simple time signature. Later composers used this device more effectively, writing music almost devoid of a discernibly regular pulse. If two time signatures alternate repeatedly, sometimes the two signatures are placed together at the beginning of the piece or section, as shown below: Additive meters To indicate more complex patterns of stresses, such as additive rhythms, more complex time signatures can be used. Additive meters have a pattern of beats that subdivide into smaller, irregular groups. Such meters are sometimes called imperfect, in contrast to perfect meters, in which the bar is first divided into equal units. For example, the time signature means that there are 8 quaver beats in the bar, divided as the first of a group of three eighth notes (quavers) that are stressed, then the first of a group of two, then first of a group of three again. The stress pattern is usually counted as : one two three one two one two three ... This kind of time signature is commonly used to notate folk and non-Western types of music. In classical music, Béla Bartók and Olivier Messiaen have used such time signatures in their works. The first movement of Maurice Ravel's Piano Trio in A Minor is written in , in which the beats are likewise subdivided into to reflect Basque dance rhythms. Romanian musicologist Constantin Brăiloiu had a special interest in compound time