Substitute by weight or volume?
So I feel like this is a dumb question, but I still don't know the answer. If I want to make a substitution in a recipe, say use some whole wheat flour instead of all white, do I substitute the flour by weight or by volume? Whole wheat flour is less dense than white, so would I replace say 100 grammes of white flour with 100 grammes of whole wheat, or would I replace 100 grammes of white flour with, say, 90 grammes of whole wheat, to try to get the same volume of flour? (I don't actually know the volume-weight conversions for the flours, but you can see what I'm asking). What about liquid ingredients?
Until recently, I've always used volume measurements in baking, and did substitutions by volume too. Now that I sometimes measure by weight, my first instinct is to make substitutions by weight, but that won't yield the same results as substituting by volume unless the things being substituted have the same density. I'm sure what's happening is that the quantities I normally work with are so small, and the ingredients to substitute are so similar in density, that it has not made a difference, but what is actually correct?