indeed. my preferred rise has about a 1 inch variance, so technically, my inseam is also pretty fixed. but since i do buy ready to wear, in reality my rise might vary by up to an inch and a half or so. because of that, and because i always wear my pants on the same point of my waist, i use the outseam when i have my tailor do my pants.
i understand i am in the minority on this one, and that most people do not treat rise/inseam/outseam the way i do.
thanks you kindly!
The rise do not affect the inseam, as our trousers should not sag or be too tight in the crotch, so always be worn in the "same" spot. So, as you buy rtw trousers, the rise may change from trouser to trouser, reflecting on the out seam thus you should specify your favored inseam to your tailor and not the out seam, as the inseam is a more precise measurement.
This is a technical matter, so I am not sure if you could explain it successfully, but that is how I approach the subject based on tailor's explanations.