This is a continuation of To pin or not to pin so you should read that first if you haven’t already. I.D. and O.D. refers to Inner Diameter and Outer Diameter. This is a common concept in every field of engineering of which sewing is one. However, it’s only in sewing that people assume that two lengths that will be joined must be identical in length. In any other field, one would be laughed out of the room for believing this. I.D is necessarily smaller than O.D., one tucks inside the other. That’s just one reason that sleeves -for example- should not be larger than the armhole to which they are sewn. Sleeves are I.D. tubes sewn into a still larger armhole tube (O.D.).
Using exactly the same pieces from the previous entry, I sewed two tubes as one would in the course of attaching cuffs or inserting sleeves. Below you can see the result of using the improperly cut lengths inserted inside each other.
As you can see, were this a sleeve, you’ve introduced a major problem were there had been none if the goods had been cut correctly. In fact, the point of O.D. and I.D. is that dependent on the weight of goods, one may need to cut the I.D. piece shorter in length than the piece it sews to anyway. Below is a photo showing the seam allowances of each piece are uniform.
In heavier weight goods, these differences are exacerbated. Most of my experience is in leather. I can promise you that using sleeve cap ease in a leather sleeve, polar fleece or in wools, is going to create all kinds of havoc. If one is accountable to 200 line stitchers who have to sew those in and your job is on the line, you’re in no position to argue regardless of myriad sewing naysayers on the web (who arguably might feel differently if they had to set 500 of the same wrong sleeve in a day). I’m not the only one who says this; most people don’t know any experienced professional pattern makers or have occasion to ask them questions. A friend with over 30 years in the business says that as a matter of course, she uses negative ease in all her sleeve caps. She thinks it’s silly that people even debate the matter.