Dead bug prototyping and freeform electronics are a way of building working electronic circuits, by soldering the parts directly together, or through wires instead of the traditional way of using a printed circuit board (PCB.)
Dead bug prototyping got its name because when you invert a IC, and bend the legs out, it looks like a dead bug. Sometimes you can make your whole circuit work just by soldering the parts directly to an IC, and the easy way to do it is to lay the chip upside down, bend the leads out and solder parts together. Sometimes people use many chips, and glue them upside down to a blank PCB, then build the circuitry from part to part.
This type of circuit is often a quick way to get going on a project, and is a good way to test stuff, before investing in printed circuit boards, which makes it useful in prototyping, or even for building small quantities, but its fairly labor intensive, often difficult to build, and ends up looking ugly.
The neat thing about building circuits without boards is that it removes the need for everything to be on a plane, making for more interesting looking 3D circuits, rather than 2D circuits. Geometry comes more into play this way, and the way parts are manufactured, the number of leads, how they are organized all limit, and shape the way the parts can fit together. Some parts have 2 axial leads, others may have 2 or more linear leads or dual in-line leads, while another may have leads in a radial fashion. Surface mount parts and through hole parts have different properties, but they can both work fine and can even be mixed in the same designs.
When I used to work building electronics, engineers would revise designs, which would cause changes in manufacturing. We would do things like clipping a lead on a part, solder a different part on to that one, or sometimes on top of another part. It was pretty common practice to piggyback memory modules or chips to make a bigger memory module or chip. This sort of thing is practical and viable in many useful ways. Its also can be aesthetically appealing, and interesting looking.
The purpose of this instructable isnt to show you how to do this, but rather that its a fun way to build interesting electronic projects, as well as a quick way to see if your thing works.
Shift registers are component often used in digital circuits, in this case, im using 74HC595 shift registers, they are very popular among hobby electrics users. They are a serial to parallel device, you send it a series of data, it outputs in a parallel fashion. Serial parts work well for piggybacking, typically the outputs are parallel, and the input is chained from one chip to the next, with several leads in common. The leads that are common, can be soldered in a direct line, that makes things easy, the outputs are also in a direct line, which makes thing nice and easy. The data chain is a simple repeating connection between one chip and another. Fitting it together in a stack isnt usually all that much of a challenge, but it can be.
Im usually using shift registers with LEDs, which means they often output to resistors, which can complicate things, but it can also make for more interesting designs. Stacking is by no means the only way to do this. but its one of the more interesting ways, particularly because vertical, is an uncommon direction to build electronics. It also illustrates how the parts work together to make one big part, like a hotel with many floors and many rooms on each floor.