Use a master cross reference light bulb guide when you need to replace a light bulb from one manufacturer with a compatible light bulb from one of that company's competitors. Finding a compatible light bulb from a manufacturer that has either gone out of business or stopped production of that particular model also requires using a light bulb cross reference guide.
Use a light bulb cross reference chart by first finding the appropriate manufacturer on the chart and then scanning that manufacturer's column for your bulb's model number or part identifier. All bulbs on the same row of the chart as your bulb are fully compatible. For example, finding the Philips PL-S9W/850 compact fluorescent bulb on a cross reference chart shows that it is fully compatible with Sylvania's S6713 CF13DS/850 bulb and General Electric's F13BX/SPX50 model.
Another reason to use light bulb cross reference charts is to take advantage of possible savings by finding compatible light bulbs from generic manufacturers. Generic brands often offer compatible light bulbs at prices below those of the major brands. Unlike major manufacturers, generic brands rarely use their own model numbers or parts identifiers and instead rely on national standards. Most smaller manufacturers, for instance, use the generic CFT13W/GX23/850 model number for the same bulb as the Philips' PL-S9W/850 and its other major manufacturer counterparts.