Why do package names contain version numbers?

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http://unix.stackexchange.com – While working with Ubuntu and other Debian-based distros, I've noticed that packages in the software repos often contain the major version number. For example, Apache: apache2 Tomcat: tomcat7 PHP: php5 Wine: wine1.4 MySQL: mysql-server-5.5 I notice however that there's no apache1 package available, and similar for the rest. If the name of the package changes with updates to the software, doesn't that get in the way of one of the major goals of package management (easy upgrades)? If Apache 3 comes out tomorrow, am I going to have to install the apache3 package manually if I want to upgrad (HowTos)