When you type you have capital letters (upper case) and small letters (lower case) so you can type A or a. So they are similar but in a separate case - so to speak. Hope I have explained this OK Squarebear.
I thought Pea Pod was a good answer as it covered the 'similarity' part of the clue, if that is what this is about. Better to add to original post really as the point gets lost whatever case you are typing in.