stand by your principles

principal and principle sound exactly the same. 

principal can be a noun or an adjective and refers most usually to the head of an organisation or a firm.

An elementary school principal has banned …

University principals 

As an adjective principal means the most important.

the principal psychologist

principle is only ever a noun and refers to a rule

the principles of design
against my principles
back to first principles

How can you remember which is which? How about a wee mnemonic (aid to memory)?

Think of your headteacher - the principal is not your pal!

Test yourself:

1. The princip… cause of obesity is 
2. I'm sticking to my principl…s
3. He doesn't have any principl…s
4. At a meeting of college principl…s  
5. The acting princip… officer is  

Answers: paste upside down answers into FLIP

 sı ɹǝɔıɟɟo lɐdıɔuıɹd ƃuıʇɔɐ ǝɥʇ ˙5  
  slɐdıɔuıɹd ǝƃǝlloɔ ɟo ƃuıʇǝǝɯ ɐ ʇɐ ˙4  
sǝldıɔuıɹd ʎuɐ ǝʌɐɥ ʇ,usǝop ǝɥ ˙3  
sǝldıɔuıɹd ʎɯ oʇ ƃuıʞɔıʇs ɯ,ı ˙2  
 sı ʎʇısǝqo ɟo ǝsnɐɔ lɐdıɔuıɹd ǝɥʇ ˙1