The technically correct answer to the poll was the draft on the left. I’m surprised more of you haven’t noticed that the answers to my quizzes are usually counter intuitive. Why else would I pick such examples except to be troublesome?
Aside from all the interesting questions posed in comments, worthy of deeper discussions that I’m not prepared to address just yet, this was a spatial test. I think most people selected the sketch on the right (b) because the pieces seemed to be aligned with the illustrated grainline. However, once you turn the pieces, you can see the grainlines of b run opposite to the illustrated “true” bias.
Definition of bias vs true bias
The designation of whether the illustrated true bias line is either 45 or 135 degrees depends on your point of reference. There are two possible starting points from which to base the angle. The visual mnemonic I use to remember the angle, facing the cut edge, is “left down, right up” with a flash visual of John Travolta in the Saturday Night Fever movie poster. It’s his right arm in the air.
I was taught that bias was a 135 degree line (not shown) and true bias was 45 degrees (dependent on my starting point). I was taught that any other variation was called “off grain”. Apparently this seems to have changed, dilution being inevitable. I don’t think it really matters but you should clearly convey your preferences when working with grey-hairs who may use a different definition.
Regarding “true” bias
I’ve decided that somebody somewhere made an arbitrary decision and since then, others have fallen in line behind it. I strongly suspect but have not proven, that the optimal bias depends on whether the threads of the fabric are an “s” twist or a “z” twist (below).
“S-twist: S- and Z-twist yarns.” Online Art. Encyclopædia Britannica Online. 16 Aug. 2007
It’s the only logical solution. Accordingly, one fabric may be better cut at the opposing bias grain than the other. Unless you examine the thread twist with a thread counter (aka linen tester), you won’t know which bias grainline is optimal.
Likewise, regardless of “s” or “z” thread twist, any garment cut on either bias grainline is going to hang better than something cut on the straight or cross grain. I doubt that in but the rarest of circumstances, would it make a noticeable difference.
That’s all for now. We can quarrel more later. I don’t have much fight in me today.
Speaking of, due to challenging personal circumstances, I’m canceling my trip to Magic. I regret if this is a disappointment but my energies are needed at home for awhile.