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Secondary III • 9mo.

What are the writing rules and standards to follow when dimensioning an object in a technical drawing?


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Explanations (1)

  • Explanation from Alloprof

    Explanation from Alloprof

    This Explanation was submitted by a member of the Alloprof team.

    Team Alloprof • 9mo.

    Thank you for your question!

    First, there are two main types of lines that are used for dimensioning in a technical drawing:

    • Dimension lines: They indicate the length of a part of an object: They indicate the length of a part of an object.

    • Extension lines: They are placed at the ends of the arrows on dimensioning lines.

    • Reference lines point to the part of a drawing to which a note or legend refers.

    • Please note that there are several other types of lines, but they are not the object of this answer.


    Certain rules must be respected when it comes to annotating:

    • You never write units in dimension numbers. These are indicated in the scale.

    • The dimensions indicated in the dimension numbers must be the actual dimensions of the object.

    • Dimensioning lines are never placed directly on the drawing of the object.

    • Dimensioning lines must always be surrounded by extension lines at their ends.

    • If the space between the dimensioning line and the extension lines is too small to draw the arrows of the dimensioning lines, it is preferable to draw these arrows on the outside of the extension lines (so that they point towards the space inside). The dimensioning figure remains between the extension lines, however.

    • The dimension number is always written on the dimension line.

    • Unless otherwise indicated, dimensions are recorded in millimeters.

    Certain rules must be respected when it comes to circular shapes:

    • The symbol ⊘ indicates the diameter of a certain hole.

    • The symbol R indicates the radius of a circle or an arc of a circle.

    Don’t hesitate to ask other questions!