Connecting Keyboards/Electric Pianos to a Mixer

[Chienmortbb]

Is there a common connector used by all standards and makes of keyboards?

[Woodinblack]

In general most keyboards will have one (or two if stereo) ¼" jack sockets. 

[nilebodgers]

Some low-end electric pianos only have a headphone socket. Plugging headphones in mutes the internal speakers, so a stereo jack to 2x mono jack adaptor allows even those to be connected to a PA.