In this section, the following term is used with the meaning indicated: "variable" (as a noun) means a feature or property (e.g., a dimension, a physical condition such as temperature, a quality such as density or colour) which, in respect of a particular entity (e.g., an object, a quantity of a substance, a beam of light) and at a particular instant, is capable of being measured; the variable may change, so that its numerical expression may assume different values at different times, in different conditions or in individual cases, but may be constant in respect of a particular entity in certain conditions or for practical purposes (e.g., the length of a bar may be regarded as constant for many purposes). Attention is drawn to the definitions of terms or expressions used, appearing in the notes of several of the classes in this section, in particular those of "measuring" in class and "control" and "regulation" in class . Classification in this section may present more difficulty than in other sections, because the distinction between different fields of use rests to a considerable extent on differences in the intention of the user rather than on any constructional differences or differences in the manner of use, and because the subjects dealt with are often in effect systems or combinations, which have features or parts in common, rather than "things", which are readily distinguishable as a whole. For example, information (e.g., a set of figures) may be displayed for the purpose of education or advertising (), for enabling the result of a measurement to be known (), for signalling the information to a distant point or for giving information which has been signalled from a distant point (). The words used to describe the purpose depend on features that may be irrelevant to the form of the apparatus concerned, for example, such features as the desired effect on the person who sees the display, or whether the display is controlled from a remote point. Again, a device which responds to some change in a condition, e.g., in the pressure of a fluid, may be used, without modification of the device itself, to give information about the pressure () or about some other condition linked to the pressure (another subclass of class , e.g., for temperature), to make a record of the pressure or of its occurrence (), to give an alarm (), or to control another apparatus (). The classification scheme is intended to enable things of a similar nature (as indicated above) to be classified together. It is therefore particularly necessary for the real nature of any technical subject to be decided before it can be properly classified. This class covers all sound-emitting devices, in general, whether or not they may be considered as being musical. In this class, the following expression is used with the meaning indicated: "musical instrument" does not exclude devices emitting a single sound signal. The following Class Index is given in place of subclass indexes, to show the grouping of the elaborations belonging to different subclasses, under the following three fundamental types: wind instruments; string instruments; percussion instruments, which relate clearly to the majority of instruments. There are of course some instruments of which the principle of operation belongs less clearly to one of the three types mentioned in Note (3). They correspond to groups or , or , all the other groups normally finding a definite place. ACOUSTICS; OPERATIONS ON SOUND WAVES Speech analysis or synthesis; speech recognition Methods or devices for transmission of sound or protection against sound, not otherwise provided for , Acoustics not otherwise provided for WIND INSTRUMENTS General features; details, accessories ; Organs, harmoniums, or similar instruments , Accordions, concertinas, or similar instruments; other types of instruments ; Whistles; sirens; horns ; ; STRING INSTRUMENTS General features; details, accessories ; Pianos, similar instruments; tools and methods for making or maintenance , ; Other types of instruments PERCUSSION INSTRUMENTS Bells, rattles, or similar instruments , Other instruments OTHER PARTICULAR DEVICES: DEVICES USING UNDEFINED PRINCIPLES; COMBINATIONS OF INSTRUMENTS; MUSIC ACCESSORIES Electrophonic musical instruments Automatic musical instruments Sirens; devices with vibrators ; Combinations: of pianos with other instruments; of other instruments ; Music accessories INSTRUMENTS NOT OTHERWISE PROVIDED FOR