Escutcheon shape?

[David]

Looking at these flags on Perfect Captain:

http://perfectcaptain.50megs.com/york.html

I was reminded of a question regarding the banner of Lord Ogle, that of the Bertram quarter.

This is or an escutcheon azure.

My interpretation of an escutcheon is that it follows the shape of the field, so in this case it should be square - which is correct?