IBM 5150 Manual To Operations page 113

Here are some filenames with extensions:
81PRICES.JUl
81PRICES.AUG
AVRG.$&%
WEATHER.80
IMPORTANT: If a filename is followed by an extension,
you
must
use both parts when telling DOS about that file.
Bad, Okay, and Good Names for Files
With all the possibilities, filenames can be unusual, to
say the least.
DOS likes names that follow the rules. These files have
names that DOS will not accept:
Name Why DOS won't accept it
A AND B Spaces in it
A,B,
&
C Commas in it
.PGM
Filename missing
ANDTHISONEISTOO.lONG
These names are okay for DOS:
o«»o.xxx
#1#2#A3B
@@.-- ­
Z
but can you guess what is in any of these files?
A good name for a file will help you remember what kind
. o f infomntion is in the file, and perhaps whether it's a
file that contains a program or only data. For example,
ADDRLJST.BAS is a good name for a BASIC program
file that prints an address list.
DOS 3-39