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Subjects · Leaving Cert Design & Communication Graphics (DCG)

Leaving Cert Design & Communication Graphics (DCG): Intersecting planes & dihedrals

How often Intersecting planes & dihedrals comes up on the Design & Communication Graphics papers, every year it was asked, and questions to try.

HL Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

Intersecting planes & dihedrals, Higher Level(7 marks)

Quick ones on Intersecting planes & dihedrals.

(a)Why can't the dihedral angle be read in the view that shows the line of intersection in true length?
  1. The line is foreshortened there
  2. The planes do not appear as edges there
  3. The heights come from the wrong view
(b)Two planes meet at a dihedral angle of 70°. What is the other angle formed between them?
  1. 290°
  2. 20°
  3. 110°
(c)A dihedral angle is measured in a plane lying in what relation to the line of intersection?
  1. Perpendicular to it
  2. At 45° to it
  3. Parallel to it
Show the answers

(a) The planes do not appear as edges there

(b) 110°

(c) Perpendicular to it

Your turn: Higher Level questions on Intersecting planes & dihedrals.

Higher Level

Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025B2
2024B1
2023A4, B2
2022B1, B3
2021A3, B2
2019A1, B3
2018A4, B3
2017A3, B2
2016B1

Links open the State Examinations Commission’s paper for that year.

More Intersecting planes & dihedrals questions

Intersecting planes & dihedrals, 2 marks

For the second auxiliary view of a dihedral angle, how is X2Y2 placed relative to the true-length line of intersection?

  1. Parallel to it
  2. Parallel to the original XY
  3. Perpendicular to it
Show the answer

Perpendicular to it

Looking straight along the true-length line shows it as a point, and both planes then appear as edges with the true angle between them.

Intersecting planes & dihedrals, 3 marks

The dihedral angle between a plane and the HP is the same as what?

  1. The angle between its HT and XY
  2. The plane's inclination to the HP
  3. The plane's inclination to the VP
Show the answer

The plane's inclination to the HP

The HT is the plane's line of intersection with the HP. Looking along it shows the plane as an edge against the HP, and that angle is both the dihedral angle and the inclination.

Intersecting planes & dihedrals, 3 marks

On a hipped roof, which line is viewed as a point to find the dihedral angle between two adjacent roof slopes?

  1. The hip
  2. The ridge
  3. The eaves line
Show the answer

The hip

The hip is where two adjacent slopes meet, so it is their line of intersection. Show it in true length, then view it end-on.

Other Design & Communication Graphics (DCG) topics

All of Leaving Cert Design & Communication Graphics (DCG)