Common Lisp arrays can be represented in a few different ways in CMU Common Lisp – different representations have different performance advantages. Simple general vectors, simple vectors of integers, and simple strings are basic CMU Common Lisp data types, and access to these structures is quicker than access to non-simple (or “complex”) arrays. However, all multi-dimensional arrays in CMU Common Lisp are complex arrays, so references to these are always through a header structure.