Dimensioning rule: Should dimension lines be arranged so they do not cross extension lines, thereby keeping the drawing legible and uncluttered?

Introduction / Context:
Clarity is the top priority in dimensioning. While complex parts sometimes force compromises, best practice is to place dimension and extension lines to minimize or eliminate crossings. This improves readability and reduces risk of fabrication errors.

Given Data / Assumptions:

• Standard line conventions and spacing are applied.
• Multiple dimensions may be required for a dense drawing.
• Repositioning or adding views is allowed to enhance clarity.

Concept / Approach:
Dimension lines carry arrowheads and values; extension lines indicate feature limits. Crossing these lines creates visual clutter and can obscure relationships between features. Good layout uses orderly dimension chains, consistent offsets, and proper grouping to avoid crossings. When congestion is unavoidable, rearrange the view, introduce a detail or auxiliary view, or stagger dimensions thoughtfully.

Step-by-Step Solution:

Verification / Alternative check:
Compare two versions of the same drawing: one with multiple dimension/extension crossings and one with clean chains. The latter will be easier to read and less error-prone, confirming the value of avoiding crossings.

Why Other Options Are Wrong:

• Crossings are not preferred; they hinder legibility.
• Coinciding lines are not allowed; each line type has its own role and spacing.
• Hidden lines are unrelated to this placement rule; clarity still governs.
• No standard mandates crossings by default; the goal is legible communication.

Common Pitfalls:
Cramming all dimensions into one view; inconsistent offsets; forgetting to use detail bubbles and auxiliary views to relieve congestion.

Final Answer:
Correct: avoid crossing dimension lines over extension lines whenever possible