A musical layer is a part of the song which contains features with a common aspect between them. A significant part of beatmapping in every game mode revolves around consistently following different kinds of layers (such as percussion, vocals, and sound patterns), and switching between them for the purpose of emphasis and contrast.
Typically, a single layer in a musical piece represents a group of instruments of the same family or range, or — alternatively — voices of the same type. Since layers are abstract, they can be limited to just a single instrument or kind of sound, depending on the context they are discussed in. Other criteria for layer subdivision include rhythm patterns, melody, harmony, or pitch.
While picking a layer to follow is usually up to a beatmap author's discretion, the choices are usually the most obvious and recognisable ones:
In addition to existing layers, mappers may arrange hit objects in such a way that creates a new layer. This approach is often called overmapping and is used for the purpose of making the beatmap more interesting due to new rhythmic opportunities, usually not present in the song. However, inconsistent or excessive note placement may make it hard for players to follow the mapper's intent, and ruin the beatmap's relation to the song or its key features.