When the first sound cards on the PC, the jacks were typically all one color, mostly black. This state of affairs remained in effect until "PC 99," a pseudo-standard collaborated on by Microsoft and Intel (and third in a series of four "PC System Design Guide" standards which also included PC 97, PC 98, and PC 2001) defined a new industry standard for sound card jack colors. The basic jack colors go something like this:
- Green: Audio out (front speakers)
- Black: Audio out (rear speakers)
- Pink: Microphone in
- Blue: Line in
- Yellow/Orange: Subwoofer out
On surround-sound sound cards, the difference between the green and the black jacks is that the green is for the front speakers, while the black is for the rear. On stereo sound cards with only one line-out jack, this jack may be either color, although green seems to be preferred as it more explicitly defines the jack as being an audio output. If a surround-sound sound card is configured for regular stereo (2-speaker) output, the green and black jacks are interchangeable. The pink and blue jacks are both used as audio inputs for the sound card to record with. The difference is that the pink jack is intended for use with an unpowered microphone, while the blue jack is meant to take a line-level audio input. In audio terms, then, the sound card will generally use a much higher-gain amplifier for the microphone input, since most computer microphones make such a weak signal (typically only a few millivolts) that the recording would be virtually inaudible without significant amplification. The blue line-level input, by contrast, will use a much lower-gain amplifier - or, frequently, no amplifier at all, as it is designed to receive an already-amplified signal produced by the output of powered audio equipment, which typically produces a signal level of over 100 millivolts. At best, plugging a line-level signal into the microphone jack is likely to vastly overdrive the sound card's microphone amplifier, resulting in unusably clipped recording; at worst, doing so may burn out the sound card altogether. Also note that since most computer microphones are mono, the pink microphone input is usually mono as well, while the blue line-in jack can typically receive stereo audio. Additionally to these colors, some sound cards emboss (or print) symbols on their mounting bracket representing the functions of these jacks for further identification. Typically, the audio-out jacks are represented with a symbol of a wave-shape with an arrow coming out of it (symbolizing sound waves coming "out" of the card), the line-in jacks have a symbol of a wave with an arrow pointing into it, and the microphone jack usually just has a small picture of a microphone. However, the surfaces of the mounting brackets on PC sound cards are so cramped for space that there is often no room for such symbols. This is also the reason why PC sound cards have standardized on the relatively tiny 1/8" (3.5 mm) jack size while most other professional audio equipment uses 1/4" (6.35 mm) jacks or RCA jacks as a standard.
