To explain a development choice, give a reason you can measure: total seam length, number of cut edges, or fit on a stated sheet. Preferences without numbers earn little.
This lesson is part of surface development concepts. It assumes your nets already pass the face count and length checks.
What can I compare?
Three measurable things: how much edge is cut and later rejoined, how big the flat shape is, and how many pieces stay attached.
Worked example: two nets for one open box
An original open-top box has a base of 40 mm by 30 mm and a height of 20 mm.
Net A: the cross. The base sits in the middle and a side is attached to each base edge. Four vertical corners are cut, each 20 mm, so the seam total is 4 × 20 = 80 mm. The bounding rectangle is (20 + 40 + 20) = 80 mm wide and (20 + 30 + 20) = 70 mm tall.
Net B: the strip. The four sides form one row, 2 × (40 + 30) = 140 mm long and 20 mm high, with the base attached to one 40 mm side. One vertical corner is cut, 20 mm.
The other three base edges are cut too: 30 + 40 + 30 = 100 mm. The seam total is 20 + 100 = 120 mm. The bounding rectangle is 140 mm by (20 + 30) = 50 mm.
Comparison. Net A has the shorter seam, 80 mm against 120 mm. Net B needs a long, thin sheet, at least 140 mm by 50 mm. Net A fits a sheet as small as 80 mm by 70 mm.
A justified answer: Net A is chosen because it needs 40 mm less seam and fits a 100 mm by 100 mm sheet, which Net B cannot, because Net B is 140 mm long.
The mistake: opinion as reason
A student writes that the cross layout is better because it is easier to fold. That may be true, but it gives the marker nothing to check.
| Statement | Type | Checkable? |
|---|---|---|
| Net A is easier to fold | Preference | No |
| Net A has an 80 mm seam against 120 mm | Geometric | Yes, by adding the cut edges |
| Net B needs a 140 mm long sheet | Geometric | Yes, from the perimeter |
A good reason names the quantity and the number. If you cannot measure it, sharpen it until you can.
When the other net wins
Net B is better if the sheet is long and narrow, such as a strip 150 mm by 60 mm, because Net A would not fit. The choice depends on the constraint in the question, not on a fixed ranking.
Check yourself
An open box has a square base of 20 mm side and height 10 mm. Net A is a cross and Net B is a strip of four sides with the base attached to one side. Compare total seam length.
Answer
Net A cuts four vertical corners of 10 mm: 4 × 10 = 40 mm.
Net B cuts one vertical corner, 10 mm, and three base edges of 20 mm each, 60 mm, so 70 mm. Net A has the shorter seam by 30 mm.
What to study next
Test your reasoning in the development practice set, and return to connecting a solid to its development if the lengths still feel shaky.
If you want a teacher to help you phrase each reason with a number, see online one-to-one Engineering Drawing tuition.