被测要素（Mcasured feature）是指图样上给出形状或（和）位置公差要求的要素，是检测的对象。
当被测要素为轴线、中心线或对称平面时，指引线的箭头和基准符号应与该要素的尺寸线对齐。
If the element being tested with tolerance requirements is an axis, centre line or symmetrical plane, the arrow and datum symbol should be aligned to the dimension line of that element.
当具有公差要求的被测要素为线或平面时，将指引线箭头和基准符号指在线或平面上，且必须与尺寸线错开。
If the element being tested with a tolerance requirement is a contour line or surface, point the arrow and datum symbol to the contour line or surface, and keep it away from the dimension line.
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