If you're not using marking gauges or some sort of blade to lay out your joinery, you are really missing out! I remember a conversation I had with William Ng at the William Ng School when he told me, “If you want to make good joinery, use a pencil. But if you want to make great joinery, using a knife!”
Marking gauges come in several forms but they are all fundamentally similar. They consist of a post, a moveable fence, and a blade or scratch pin. My personal preference is for blades and you can see a fairly standard traditional marking gauge to the left. So why should you use one? Keep reading.
When you cut with a knife, the resulting line is maybe a couple thousandths of an inch wide. Compared to a big chunky pencil line, its much easier to know when you're exactly where you need to be in reference to that line. Additionally, marking gauges have fences which allow us to be consistent when marking multiple sides of a single workpiece for dovetails and tenons.
No doubt you are very familiar with the concept of tearout. When you cut wood across the grain, you'll inevitably notice small bits of wood tearing out at the end of the board where the fibers are unsupported. So if you actually cut that grain ahead of time with a blade and then cut right up to the line with your saw, you end up with a nice clean crisp shoulder with absolutely no tearout.
If you use scribe lines, you'll have a perfect place to lay the tip of your chisel or saw blade with absolute accuracy and consistency. If you try to line up the tool with a pencil line, you will almost always end up on one side of the line or the other. There's just too much variability there. But if you have a little trough from a marking gauge or knife, you'll have a no-brainer aid for locating the tool in the proper place.