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 subclass covers mechanically-driven calendar clocks or clockwork calendars, and the mechanical part of such clocks or calendars. DRIVING MECHANISM WINDING Normal; automatic; combined ; ; Supervision; winding parts ; CLOCK MOVEMENT Escapement; frequency stabiliser; setting frequency gearwork; adjusting thereof ; ; ; ; TIME INDICATING , , , TIME SETTING FRAMEWORKS; SUPPORTS; CALIBERS ; ; PROTECTION OF CLOCKWORK Cases; crystals, glasses; other protection means ; ; , UNUSUAL CLOCKS , , SUBJECT MATTER NOT PROVIDED FOR IN OTHER GROUPS OF THIS SUBCLASS This subclass covers electric features of mechanically-driven clocks or watches, such as electric winding of such clocks or the provision of electric contacts thereon. ELECTRIC WINDING OF MECHANICAL CLOCKS ELECTROMECHANICAL CLOCK MOVEMENTS; ELECTRIC OR MAGNETIC ESCAPEMENTS , TIME INDICATING Optical; acoustical means , ; TIME SETTING POWER SUPPLIES SYNCHRONISATION; MASTER-AND-SLAVE CLOCK SYSTEM; SYNCHRONOUS-MOTOR CLOCKS ; ; CLOCKS FOR OPERATING A DEVICE AT A PRESELECTED TIME SUBJECT MATTER NOT PROVIDED FOR IN OTHER GROUPS OF THIS SUBCLASS HAND AND MACHINE TOOLS , LUBRICATING DEVICES MEASURING AND TESTING APPARATUS DEMAGNETISING DEVICES SUBJECT MATTER NOT PROVIDED FOR IN OTHER GROUPS OF THIS SUBCLASS This subclass covers: apparatus for measuring-off predetermined time intervals; apparatus for producing such intervals as timing standards, e.g. metronomes; apparatus for measuring unknown intervals, e.g. precision systems for short-time-interval measurement. MEASURING PREDETERMINED TIME INTERVALS Producing time standards Apparatus: without driving mechanisms; with driving mechanisms ; MEASURING UNKNOWN TIME INTERVALS Mechanically; electromechanically; electrically; otherwise ; ; ; This subclass covers: electronic time-pieces with no moving parts; electronic circuitry for producing timing pulses irrespective of the nature of the time-indicating means utilised. This subclass does not cover electronic time-pieces with moving parts, which are covered by subclass . PRODUCING TIMING PULSES TIME-SETTING; SYNCHRONISING ; TIME- OR DATE-INDICATING Visual; optical signals; acoustic signals ; ; OPERATING A DEVICE AT PRESELECTED TIMES STRUCTURAL DETAILS; HOUSINGS ELECTRIC POWER SUPPLY CIRCUITS OTHER SUBJECTS