Vector of Vectors possible?

Looking at some stan code from a package, I see a data format that appears to violate the rules:

vector[n] x[t];

Is this some kind of “vector of vectors”? (My understanding was that vectors are single columns) Or, is this some kind of awkward matrix?

Thanks in advance!

This is an older way of writing array[t] vector[n] x;. Hopefully the newer syntax makes it more clear what this is: an array of t vectors, each with length n.

This is not quite equivalent to a matrix, since it will be laid out differently in memory. Some details are described in the User’s Guide

Ahh, that makes sense.

Thank You!!!