In the previous posts on gauge, I've mostly focused on gauge over stockinette swatches. There's a reason for that, of course; that's by far the most common fabric over which gauge is measured. This is the case even for many projects which aren't worked in stockinette; designers will quite often make the assumption that if you match gauge in stockinette, you'll also match it in the pattern stitch. It's not an unreasonable assumption. However, sometimes the gauge is given over the pattern stitch, or you'd just like to check your gauge in pattern against the finished size, or perhaps you're designing something of your own and want to predict the finished size of the full item. We'll talk about a couple of common situations: measuring gauge over ribbing, and measuring gauge over cables, lace, and other irregular textures.
Counting stitches over ribbing is pretty easy, and the stitches are fairly uniform in size; also, the fabric is balanced, so you don't need borders on your swatch to keep it from rolling up (although putting perhaps one or two garter-stitch stitches on each edge, so the first and last ribs aren't edge stitches, might be helpful, and the other guidelines such as using the same method and taking pre- and post-wash measurements would still apply). The issue with this fabric is how stretched or relaxed it's supposed to be when you take the measurement -- many patterns specify "slightly stretched" or some similar language, which is subject to all kinds of interpretation. This is a fairly subjective area, with individual taste coming into play, but hopefully I can provide some useful guidance.
The key to evaluating gauge over ribbing is to evaluate the fabric as a whole: do you get gauge when the fabric is stretched to a degree that will be attractive when it's in use? There are two approaches to this -- either stretch the fabric to a state you like and then count stitches, or stretch it to the measurements you want and then consider whether you like the fabric in that state. You can use whichever approach you prefer; I like the first one when I'm designing and trying to decide what my own gauge is, but find the second somewhat easier when I'm trying to match gauge for a pattern. We'll look at the mechanics of both.
For either of these methods, you're going to want something to pin to and enough pins to hold down your fabric without scalloping the left and right edges. For the evaluate-then-measure method, pin down one side edge, and then stretch the fabric out to a degree that looks good to you, and pin down the other side. Don't pin the top and bottom edges. Measure across 4" in the middle of the fabric, and count the number of stitches (including fractions). Ribbing generally contracts a bit vertically when it's stretched horizontally, so you'll need to look at that; measure vertically toward the center of the fabric, away from both pinned edges. Unpin the fabric, and measure vertically in the relaxed state; this state is how the fabric will appear when you're knitting, but it will pull up to the prior measurement when worn -- account for this if the pattern asks you to knit to a specific length.
For the measure-then-evaluate method, it's useful to have a paper template of a 4"x4" square. Pin down one edge of your fabric, line the paper template up with that edge, and then stretch the fabric so that the appropriate column of stitches lines up with the other edge of the template, and pin it down. Your fabric now matches the gauge -- do you like it? Is it overstretched, too loose, or just right? If you like it, you're done; if it's overstretched, it's too small and you should go up needle sizes, while if it's too loose, it's too big and you should go down. As with the other method, you'll want to evaluate any drawing up as a result of the horizontal stretch.
You can see that there's a pretty strong subjective element with both of these methods; unlike a stockinette swatch, a ribbed swatch doesn't have one right answer to "what's the gauge?". It's entirely possible, for instance, that one knitter could decide that a swatch looks great when stretched to 22 stitches over 4", and another knitter could decide that a swatch of exactly the same size is too stretched at that gauge, and that bigger needles are needed. However, in the end, both of these knitters should end up with finished items that make them happy.
Measuring gauge over cables, lace, and other textures is slightly less subjective than measuring over ribbing, but it's still got some tricky aspects. One of the complicating factors is that stitches are often not the same size consistently throughout the swatch -- some are stretched out, some are drawn in, some patterns make the rows slant up or down, and that makes just counting them a difficult proposition. Another factor, which does contain a more subjective element, is that these patterns are often intended to be blocked, but the degree of blocking is subject to the knitter's preferences.
A method for dealing with the inconsistencies between stitches in the textured patterns is to use an average gauge for one or more repeats, and convert that to the equivalent of 4" worth of stitches, rather than trying to measure 4" on the swatch. As an example, let's say that you're working on a lace pattern, and the gauge given is 16st/4" in the blocked pattern, but your pattern repeat is 12 stitches. You can't do a pattern and a third, and you really wouldn't want to try. What you want to do instead is cast on about 32 stitches, which gives you enough for a 3-stitch border plus one stockinette stitch, on either side of 2 pattern repeats in your lace pattern. Work around 4 inches of your pattern (and preferrably at least 2 vertical repeats of the lace), using bottom and top borders as with any other swatch. Measure your gauge by measuring from the inside edge of one stockinette edge stitch to the inside edge of the other one (if your pattern varies the number of stitches per row, or if the sides aren't straight, then you'll want to mark the point in the center of the swatch where you go from one pattern repeat to another, and take your measurement there), and then divide 24 by that number to get stitches per inch, and then multiply by 4 to get stitches over 4 inches. This gives you a pre-blocking measurement, useful for comparing as you knit. Now, since you're trying to match a particular gauge, you'll want to block to size -- 16st/4" is 4 stitches per inch, and 24 pattern stitches divided by 4 stitches per inch gives you 6 inches, so that's the width you want to block to. If you were instead designing with the lace, you'd want to block to the degree that seemed good to you, and then measure the resulting gauge and convert it to a 4" equivalent just as you did with the pre-blocking gauge.
What about vertical gauge? This can really be tricky in some lace patterns, where offset increases and decreases cause the fabric to bias, and makes it difficult to tell if you're measuring between the same number of rows at all points. I'd suggest placing a line of sewing thread, in a contrasting color, through all the stitches of the first row of the lace pattern, and again in all the stitches of the first row of a subsequent repeat, at least 4 inches and 2 pattern repeats away (if you only want to knit 2 repeats, knit just the first row of a third repeat as the final row of your swatch). This will let you measure the vertical distance between points that you know to be the same part of the repeat, and over a known number of rows, so you can take an average measurement as you did with the width.
Finally, a note about measuring the overall length of an item when a lace pattern has caused the bottom edge to scallop, and a pattern directs you to knit to a given number of inches. As a general matter, you're going to want to measure from the highest point of the scallop to the needle; measuring from the lower point is going to give you fabric that's too short in places, leading to unintended peekaboo effects.
Unfortunately, the rules for measuring gauge over these specialty fabrics just aren't, and can't be, as clear as those for measuring over stockinette; personal judgment plays a much larger part. You can see why designers are often happier to provide a stockinette gauge swatch! However, it's my hope that this will at least give you some guidance and a place to begin, on which to exercise your judgment in a way that's helpful to you.