No offense taken, I have twice un-deaded your comments because you seem to be posting in good faith :) Again, you get used to it. Note, the underscore would re

No offense taken, I have twice un-deaded your comments because you seem to be posting in good faith :) Again, you get used to it. Note, the underscore would represent a subscript (in the standard math-writing tool of LaTeX, but also in many variants of Markdown). Here, d stands for dimension. If you talk about dimensionality a lot, this is one of a few common letters to use to describe it (others popular contenders, especially when speaking about pairs of dimensions would would be n and m, p and q, r and s. But N is already used in a different context [weirdly n/N seems to be an exception to the following paragraph]). And since there are a few different dimensions floating around, we label them all with d, and give them subscripts to indicate which one we're talking about. d_f symbolically means "dimension of the feature space", d_k means "dimension of the key space" (also the same as the query space. in my description I called them question/answer instead of query/key). d_v means "dimension of the value space." In many contexts, the use of capital letters implicitly denotes matrices. So if you have a matrix that turns x (your input variable), into q (a query vector), naming it Q makes a lot of sense. Especially when we have three matrices floating around, it would have been much more confusing to keep track of if we named the matrices M, N, O. Which one projects to which vector? Much easier to understand that Q makes the q vectors, K makes the k vectors, V makes the v vectors.