Project Planning, Design & Implementation — Engineering Drawings and its Concepts, NEC licence examination syllabus (Nepal Engineering Council).
Line Diagram
Ten line types, each carrying a different meaning — the vocabulary a drawing is written in.
🌍 Where this lives: A drawing is read by people who never meet its author, often years later and often in another country. That only works because the line types are standardised: a dashed line means "hidden edge" to a machinist in Kathmandu, Stuttgart and Osaka alike, with no translation. This is one of the oldest and most successful universal notations in engineering — older than SI units in industrial use. Search "ISO 128 technical drawing line types conventions".
The line types
EVERY LINE ON A DRAWING CARRIES MEANING THROUGH THREE
PROPERTIES: ITS STYLE (continuous, dashed, chain), ITS THICKNESS
(thick or thin), AND ITS APPLICATION.
THE STANDARD SET, per ISO 128:
1. CONTINUOUS THICK ──────────────── (0.5–0.7 mm)
VISIBLE OUTLINES AND EDGES. The most important line on the
drawing, and drawn heaviest so that the object's shape
reads instantly from across a room.
Also used for: visible intersection edges, and the sheet
border.
2. CONTINUOUS THIN ──────────────── (0.25–0.35 mm)
THE WORKHORSE, with the widest range of uses:
· dimension lines and extension lines
· leader lines (pointing to a feature with a note)
· HATCHING (section lining)
· outlines of adjacent parts
· short centre lines
· imaginary intersections
· the diagonals marking a flat surface on a shaft
3. CONTINUOUS THIN WAVY ∿∿∿∿∿∿∿∿∿
IRREGULAR BOUNDARY — a break in a view, shown when only
part of a long uniform object is drawn. Freehand.
4. CONTINUOUS THIN WITH ZIGZAG ─/\─/\─
LONG BREAK LINE, the ruled alternative to the wavy line,
used for long straight members and preferred in CAD (which
cannot draw freehand).
5. DASHED THIN ─ ─ ─ ─ ─ ─ ─ ─
HIDDEN EDGES AND OUTLINES — features present in the object
but concealed behind material in this view. Dashes about
3 mm with 1 mm gaps.
THIS IS THE LINE THAT MAKES ORTHOGRAPHIC PROJECTION WORK:
it lets a single view report what lies behind the surface.
6. CHAIN THIN ─ · ─ · ─ · ─ · ─
CENTRE LINES — axes of symmetry, centres of circles, paths
of motion, pitch circles.
Long dash, short gap, dot, short gap, long dash.
7. CHAIN THIN, THICK AT ENDS AND CHANGES OF DIRECTION
━ · ─ · ─ · ━
CUTTING PLANE — shows where a section has been taken. Ends
carry arrows indicating the direction of view, and letters
(A—A) matching the sectional view's title.
8. CHAIN THICK ━ · ━ · ━ · ━
INDICATES A SURFACE REQUIRING SPECIAL TREATMENT — heat
treatment, plating, a specified finish. Drawn alongside the
surface, not on it.
9. CHAIN THIN DOUBLE-DASHED ─ · · ─ · · ─
ADJACENT PARTS, EXTREME POSITIONS OF MOVING PARTS, the
outline before forming, and parts in front of a cutting
plane. Also called a phantom line.
10. CONTINUOUS THIN STRAIGHT WITH ZIGZAG — see 4.
THE THICKNESS RATIO: THICK LINES ARE TWICE THE WIDTH OF THIN
ONES. The pairs come from the √2 series of the previous topic —
0.25/0.5, 0.35/0.7, 0.5/1.0 — and one pair is chosen for the
whole drawing according to sheet size and reduction.
PRECEDENCE — WHICH LINE WINS WHEN TWO COINCIDE. This is a
guaranteed exam question and is decided by information content:
1. VISIBLE OUTLINE (continuous thick) — beats everything
2. HIDDEN LINE (dashed)
3. CUTTING PLANE
4. CENTRE LINE
5. extension / dimension lines
THE REASONING: a visible edge is a fact about the object's
surface; a centre line is a construction aid. WHEN THEY
COINCIDE, THE FACT IS DRAWN AND THE AID IS OMITTED — you never
lose real information to keep a helper line.
THE DRAWING CONVENTIONS FOR DASHED AND CHAIN LINES, where marks
are commonly lost:
· A DASHED LINE STARTS AND ENDS WITH A DASH, never a gap.
· Where a hidden line meets a visible one, the dash TOUCHES
the visible line.
· Where two hidden lines meet at a corner, THE DASHES MEET
at the corner.
· A hidden line that CONTINUES a visible line leaves a GAP
at the junction, so the change of status is visible.
· A CENTRE LINE CROSSES AT THE DOT/DASH INTERSECTION at the
centre of a circle, and EXTENDS SLIGHTLY BEYOND the
outline — about 3 mm.
· Centre lines are not drawn through the space between
views.
Lettering, and the line diagram in engineering practice
LETTERING — the drawing's other notation, and standardised for
the same reason:
· SINGLE-STROKE GOTHIC style; no serifs, no shading.
· CAPITALS ARE PREFERRED throughout. Lower case is used only
for unit symbols where the standard requires it (mm, kg).
· Heights from the standard series: 2.5, 3.5, 5, 7, 10, 14 mm.
MINIMUM 2.5 mm, because anything smaller becomes illegible
after a drawing is reduced or microfilmed.
· Vertical or inclined at 75°, but ONE STYLE THROUGHOUT A
DRAWING.
· Spacing between lines of text ≥ 70% of the character height.
· TITLES ARE LARGER than dimension values, which are larger
than notes — the hierarchy tells the reader what to read
first.
THE TERM "LINE DIAGRAM" IN ITS OTHER SENSE, which the syllabus
also intends:
A LINE DIAGRAM IS A SIMPLIFIED REPRESENTATION IN WHICH
COMPONENTS ARE SHOWN AS SYMBOLS AND CONNECTIONS AS SINGLE
LINES, WITH NO ATTEMPT AT PHYSICAL SHAPE, SIZE OR POSITION.
IT ANSWERS "WHAT IS CONNECTED TO WHAT", NOT "WHERE IS IT AND
WHAT DOES IT LOOK LIKE" — and it is deliberately not to scale.
THE MAJOR KINDS AN ENGINEER MEETS:
SINGLE-LINE (ONE-LINE) DIAGRAM — electrical power systems.
A three-phase circuit is drawn as ONE line, with standard
symbols for transformers, circuit breakers, busbars and
loads. Every substation in the country is documented this
way, because drawing three conductors would treble the
clutter and add nothing.
CIRCUIT / SCHEMATIC DIAGRAM — electronics. Components as
symbols, connections as lines. The physical board looks
nothing like it, and that is the point: the schematic shows
FUNCTION, the PCB layout shows PLACEMENT.
PIPING AND INSTRUMENTATION DIAGRAM (P&ID) — process plant.
Every vessel, pump, valve and instrument, with its tag
number. The master reference document for a chemical or
water treatment plant.
FREE BODY DIAGRAM — structural analysis. The member as a
line, with forces and reactions as arrows. THE ENTIRE
DISCIPLINE OF STRUCTURAL ANALYSIS OPERATES ON LINE
DIAGRAMS: a beam is a line, a truss is a set of lines
meeting at nodes, and the real cross-section enters only
later through its area and moment of inertia.
BLOCK DIAGRAM — systems. Functional blocks and signal flow.
WHY THE ABSTRACTION IS AN ADVANTAGE RATHER THAN A LOSS:
A line diagram DISCARDS EVERYTHING IRRELEVANT TO THE
QUESTION BEING ASKED. A structural engineer analysing a
truss does not need to know the bolt pattern at each joint
to compute the member forces — and including it would make
the diagram unreadable. THE SKILL BEING TESTED IS KNOWING
WHAT TO LEAVE OUT.
The corresponding danger is that the abstraction hides a
real effect: a truss line diagram assumes pin joints, and
the real welded joints carry moments the diagram cannot
show. AN ENGINEER MUST KNOW WHAT THEIR DIAGRAM HAS
DISCARDED.
Precedence is decided by information content, and that single principle answers every case: a visible outline states a fact about the object's surface, while a centre line is a construction aid. Where they coincide the fact is drawn and the aid omitted — real information is never sacrificed to keep a helper.
🌍 Go further: The free body diagram is worth recognising as the same idea applied to analysis rather than manufacture. A structural engineer analysing a bridge truss replaces every member with a line and every joint with a point, then solves for member forces — the whole discipline runs on this abstraction. Its power and its danger are the same thing: the line diagram assumes pin joints that transmit no moment, while the real welded joints do transmit moment, so the diagram's answer is approximate in a known direction. A competent engineer can always say what their diagram discarded and whether it mattered. Search "free body diagram idealization structural analysis assumptions".
💡 Exam angle: tabulate the line types with style, thickness and application — continuous thick for visible outlines, continuous thin for dimensions and hatching, dashed for hidden edges, chain thin for centre lines, chain with thick ends for the cutting plane. State that thick is twice thin. The precedence order (visible → hidden → cutting plane → centre → dimension) is very frequently asked, so give the reasoning as well as the list. Know the drawing conventions: dashed lines start and end with a dash, dashes meet at corners, centre lines extend about 3 mm past the outline. For lettering, quote single-stroke gothic capitals and the 2.5 mm minimum height.
Syllabus points
Types of lines and their use
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