Dropping the 'e' when you add 'ing'

[Unregistered]

Could anyone please tell me what the rule is called when you drop the 'e' off the end of a word to add 'ing' - such as the word having. I'm sure I learnt at school that this rule is called something specific, but cannot remember. Many Thanks Tess.

[Casiopea]

It's called, the Silent E spelling rule.

Silent E Spelling Rule
Drop the -e, and add -ed or -ing.