Compare two txt files

Hi, I want to compare two txt files and output the matches to a file. For example I have
FILE1 - accounts1.txt
Peter
norman
elektra
table
CHAIR
Newman
nOvice
FILE2 - accounts2.txt
noland
trainfil
peter
Newman
norman
neverlan
CHAIR
jackson
KelSo

I want a line by line comparison that is not case sensitivie and want the output in file MATCHES:
Peter
norman
CHAIR
Newman

Thanks for your help.

man diff

man grep

grep -if file2 file1
sort -f file1 file2 | uniq -id

Thank you this worked.