Identifying a capacitor in a circuit in the 1950s was easy. Circuits were made of only a few different kinds of components, and capacitors were clearly marked. Today, not only are there many more kinds of parts on a printed circuit (PC) board, they also may be hard to tell from one another. Fortunately, both PC boards and their parts are printed with codes to help you identify them.
Most commercial printed circuit boards get silk-screened lettering before the manufacturer adds parts. The printed lettering may include the company name, logo, model number, and the locations of major components. Designers note capacitor locations with a "C" followed by a consecutive number. So the first two capacitors will be labelled "C1" and "C2." These markings make manufacturing and repair easier, since you can identify particular components more easily.
You'll find most of the silk-screened markings on the component side of a circuit board. All the soldered components go on one side of the board, and the markings can identify each part. Engineers carefully lay the circuit board out when it's first designed, and locate the parts and lettering so you can still see the silk-screening when the parts are installed. The parts do not obscure the lettering.
Traditional capacitors come packaged as cylinder-shaped components with two connecting leads. The axial layout has one lead coming out each end, on a radial type, both leads come straight out of one end. In most cases, the capacitor has its values stamped on it for easy identification. Markings include the manufacturer's name, the capacitor's capacitance value, and its voltage rating. You can identify capacitors like these easily because of their shape, size and markings.
More recently, manufacturers introduced surface-mount parts, including capacitors. These parts are rectangular and flat, meant to mount to the top side of a circuit board. Surface mount parts are very compact, increasing component density and assembly speed for high-volume manufacturing. Since these parts are so small, they may or may not have any printed numbers, like a traditional capacitor would.
Some capacitors have polarity, that is, a positive and negative side. The PCB manufacturer must install the capacitor in the correct way or risk damaging both the part and the circuit. Traditional capacitors have markings, such as a minus sign, to indicate polarity. The circuit board itself also carries silk-screened marks to show a positive or negative side. In addition, the bottom of the board may also carry a plus sign etched in copper, near the capacitor's positive connection.