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In every such framework we imagine three surfaces perpendicular

to each other marked out, and designated as "co-ordinate planes"

("co-ordinate system"). A co-ordinate system K then corresponds

to the embankment, and a co-ordinate system K′ to the train. An

event, wherever it may have taken place, would be fixed in space

with respect to K by the three perpendiculars x, y, z on the

co-ordinate planes, and with regard to time by a time value t.

Relative to K′, the same event would be fixed in respect of space

and time by corresponding values x′, y′, z′, t′, which of course

are not identical with x, y, z, t. It has already been set forth

in detail how these magnitudes are to be regarded as results of

physical measurements.

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