A scale is a marked line with ticks and numbers: you read a quantity from it (length in centimeters, temperature, time…). What matters is the zero (or other starting value), the direction numbers increase, and the unit between neighboring marks. A coordinate ray is a ray with chosen origin OO, direction, and a unit segment (the distance from 0 to 1). Each point on the ray is matched with a number — its coordinate. The number 0 is the coordinate of the origin; then come 1, 2, 3, … A point with coordinate aa may be written A(a)A(a) or briefly “point aa.”
Scale: ticks, numbers, unit
On a coordinate ray — origin O and unit from 0 to 1
Coordinate of a point — the matching number
Addition and subtraction can be shown as steps along the ray
Scale in centimeters
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