When a DOS program prints, it usually passes the information to be printed to one of these three ports – LPT1 is used unless you specify a different port (LPT1 is the default). If you have more than one printer, LPT1 is usually the port you use for your primary printer.
The confusing thing is that the names LPT1, LPT2, and LPT3 aren’t permanently linked to the actual, physical ports on your computer. Almost every DOS computer has at least one parallel port, and initially, LPT1 is identified with one of these ports. Using DOS commands, however, you can change things around so that when your program sends something to LPT1, DOS sends that information to one of your serial ports instead. This is called “redirection.”
Why would you want to do this? Actually, it’s not often very useful, especially now that the software we have is getting smarter.
But here’s a possible scenario: Let’s say you’re using one of your serial ports to connect your computer to a network (a collection of computers and other devices all wired together). On your desk, you have a funky dot matrix printer connected to your pc’s parallel port, but there’s a beautiful high-resolution laser printer out there on the network. The dot matrix machine is OK for printing draft copies of your document. But when it’s time to print the final masterpiece, you redirect LPT1 to that serial port, and now you can print on the laser printer, without changing the port your software uses (you’d still have to change the printer driver, and that’s probably more trouble than changing the port).