Orthographic Projection: First Angle vs Third Angle

A part exists in three dimensions; a sheet of paper has two. Orthographic projection is the method that bridges them. You look at the part square-on from several directions and draw what you see, each view on its own place on the sheet. Put the views together in your head and the part comes back.

“Orthographic” means the lines of sight are parallel and perpendicular to the projection plane. Nothing is foreshortened, nothing gets bigger because it is nearer. That is exactly what makes the drawing measurable.

The six principal views

Imagine the part inside a glass box. Project it onto each of the six faces and then unfold the box flat. You get six views: front, top, bottom, left, right and rear.

Figure 4.1 — The six principal views and their layout in first angle projection

The front view is the anchor. Everything else is positioned relative to it.
Choose as the front view the face that tells you most about the part — usually the one showing its characteristic shape, with the part in its working position.
Draw the fewest views that fully describe the part. Extra views are not thoroughness, they are clutter.
Most parts need two or three. A flat plate may need one view plus a thickness note; a turned part may need one view plus Ø symbols.

First angle and third angle

There are two accepted ways of unfolding that glass box, and they place the views in opposite positions. This is the single most dangerous thing to get wrong on a drawing.

Figure 4.2 — Symbols and view layouts of first angle (European) and third angle (American) projection

	First angle (Europe, Türkiye)	Third angle (USA, Canada, Japan)
View from above	drawn below the front view	drawn above the front view
View from the left	drawn on the right	drawn on the left
Principle	the part is projected through onto the far plane	the plane is between viewer and part
Symbol	truncated cone, small circle on the right	truncated cone, small circle on the left

Always check the symbol first. The projection method is stated in the title block by the truncated cone symbol. If you read a third angle drawing as first angle, every asymmetric feature ends up on the wrong side — you produce a mirror image of the part, and it will not assemble.

Deriving views from the part

Figure 4.3 — A pictorial of an L-block and the three views derived from it

The views are not three independent pictures — they are locked together. Any height you can measure in the front view appears at exactly the same height in the side view. Any width in the front view appears at the same width in the top view. This alignment is the backbone of orthographic projection and it is what lets you check a drawing for errors.

How to work through it

Choose the front view and place it.
Project construction lines horizontally to place the side view, vertically to place the top view.
Transfer depth between the top and side views (45° mitre line or compass).
Draw visible edges thick, hidden edges dashed, centre lines chain.
Check alignment: every feature must line up across all views.

Remember that an edge hidden in one view is usually visible in another. If a feature is dashed in every view you have drawn, you are probably missing a view — or a section, which is the subject of the next lesson.

Worked Example: L-Block with a Through Hole

The part below is the exam classic of projection logic: an L-shaped block with a hole drilled straight through from front to back. Build the three views in your head before looking, then compare. The critical points: the hole is a circle in the front view but two parallel dashed lines in the top and side views; in the side view the step edge lies behind the full-depth tall wing, so it is drawn as a hidden edge; centre lines appear in every view as chain lines.

Test yourself

In first angle projection, where is the view from above drawn?
Below the front view.
In third angle projection, where is the view from the left drawn?
On the left.
How do you know which method a drawing uses?
From the truncated cone symbol in the title block.
How many views should a drawing have?
The fewest that fully describe the part — usually two or three.
Which face should be the front view?
The one that best describes the part, with the part in its normal working position.
A height measured in the front view — where else does it appear?
At exactly the same height in the side view; the views are aligned.
What happens if you read a first angle drawing as third angle?
You get a mirror image of the part.

Next lesson. Views show outside surfaces. When a part has internal features — bores, pockets, cavities — dashed lines quickly become unreadable. Lesson 5 covers section views: how to cut a part open on paper, how to hatch it, and which parts are never hatched.