# Understanding Measurement of length

Length
measurement is implemented in practice in many ways. The most commonly used
approaches are the transit-time methods and the interferometer methods based

upon the speed of light. For objects such as crystals and diffraction gratings,
diffraction is used with X-rays and electron beams. Measurement techniques for
three-dimensional structures very small in every dimension use specialized
instruments such as ion microscopy coupled with intensive computer modeling.
Standard
rulers

The ruler the
simplest kind of length measurement tool: lengths are defined by printed marks
or engravings on a stick. The meter was initially defined using a ruler before
more accurate methods became available.

Gauge blocks
are a common method for precise measurement or calibration of measurement
tools.

For small or
microscopic objects, microphotography where the length is calibrated using a
graticule can be used. A graticule is a piece that has lines for precise
lengths etched into it. Graticules may be fitted into the eyepiece or they may
be used on the measurement plane.

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