An edge is visible if you would see it from the direction of the view, and hidden if another part of the object stands in front of it. Visible edges are solid and hidden edges are dashed.
This lesson belongs to orthographic views. If the line types themselves are unfamiliar, start with interpreting line types.
What question decides each line?
For every edge, ask: “from where I stand for this view, is anything in front of it?” The answer gives the line type. You do not need to remember rules for each view, only to ask the question.
Worked example: a block with a blind hole
The object is a rectangular block 60 mm wide, 40 mm deep and 30 mm high. A round hole of diameter 20 mm is drilled from the top face, centred, to a depth of 18 mm. It does not go through.
Top view. You look straight down the hole. The circle of the opening is visible, so it is drawn solid, with a centre cross. The flat bottom of the hole has the same circle, so nothing extra is added.
Front view. The outside is a rectangle 60 × 30. The hole is inside the block, so both of its sides are hidden.
Two dashed vertical lines are drawn at 20 mm and 40 mm from the left edge, running from the top edge down 18 mm. A dashed horizontal line joins their lower ends. A centre line runs down the middle.
Side view (from the left). The outside is a rectangle 40 wide (depth) and 30 high. The hole is centred in the depth, so the dashed lines are at 10 mm and 30 mm from the left edge of this view, again 18 mm deep.
Check the numbers. The hole is 20 mm across, centred in 60 mm, so its edges are at (60 − 20) ÷ 2 = 20 mm and 40 mm. In the 40 mm depth, they are at (40 − 20) ÷ 2 = 10 mm and 30 mm. The depth 18 mm is the same in both views.
What happens when lines overlap?
If a hidden edge would fall exactly under a visible one, draw the solid line only. A dashed line under a solid line adds nothing and cannot be seen.
If a hidden line and a centre line fall in the same place, draw the dashed line. Show the centre line only where it extends past it.
The mistake: drawing the hole solid in the front view
A student draws the two vertical lines of the hole as solid because “the hole exists”.
| Decision | Wrong | Right |
|---|---|---|
| Hole in the front view | Solid lines | Dashed lines |
| Reason given | The hole exists | The front face is in front of the hole |
| What it shows | An opening in the front face | A hole inside the block |
Solid lines here say the front face has an opening, which describes a different object.
Check yourself
A block is 50 mm wide, 30 mm deep and 20 mm high, with a square notch 10 × 10 mm cut along the full depth of its top left edge. In the front view, is the notch’s inner corner edge solid or dashed? In the top view, is the notch’s edge solid or dashed?
Answer
A notch cut along the full depth of its top left edge is open to the left and to the top.
In the front view you see its profile directly, so the notch edges are solid. In the top view, looking down, you see the notch floor and its inner edge directly, so those are solid too.
Nothing is in front of either, so nothing is dashed.
What to study next
Use shared dimensions to check your views in checking alignment and dimensions across views, then try the orthographic views practice set.
If you want a teacher to question every dashed line on your own drawings, see online one-to-one Engineering Drawing tuition.