In the Real World Examples
Let's say you're a big, BIG fan of Barry Manilow. Don't worry, we won't say it to anyone else; the point is just to say it in this example. You have a collection that consists of vinyl records, 8 track tapes, cassettes, and CDs. You want to place the data about your collection into a matrix.
Let's say we want to be more detailed about our Manilow Mania. We want to list in a matrix how many of each "Greatest Hits" (1978) and "Ultimate Manilow" (2004) we have. Of the eight records there are five and 3 respectively; for 8 tracks it's 4 and 2, for cassettes 2 and 5, and for CDs it's 1 and 2.
Finally, let's suppose we bought some of each of these works on eBay and some at Julio's Retro Wonderland, and for some reason, this matters to us. For sanity's sake, let's use this shorthand: G = Greatest Hits, U =Ultimate Manilow; and E = eBay and J = Julio's place. Here are our lists:
Matrix E (our eBay purchases)
Vinyl records: 3G, 3U
Matrix J (our Julio's bargains)
Vinyl records: 2G, 0U