Programmers should make careful use of version numbers so they know that they are always maintaining the 'latest' version.

Every time a change is made the version number should be incremented and noted.

In practice a standard has developed where version numbers are in two (or more) parts. Eg Version 3.8

When a small change is made, the second number is incremented (to Version 3.9). The first number only changes on major re-writes (to Version 4.0).