> Hm. Weird. Never seen that before. Let's check the source, emilypost.com

This is taught to children in the United States. I didn't go to an especially good school and I remember it to this day. If you're not from the US the custom may be different.

Perhaps you will consider the advice from Grammarly [0] as a more acceptable source.

[0] https://www.grammarly.com/blog/commonly-confused-words/ms-mr...