Naming convention for properties in property file

As per my understanding, there is no any standard rule written for .properties file in java. but if you see the .properties files inside the lib folder of Java\jre most of them have lower cased .properties file names. And the properties themselves are also lower cased. such as:

psfontj2d.properties

key values

courier_new=courier
courier_new_bold=courier_bold

Naming convention is recommended as lowercase in property file. bank.account.number this is more appreciable.