I'm lying down, with a dah-ditty-dah, in the middle of the end,
I'd hang upside-down similarly with my friend,
Were I to flip over, with a dah-ditty-dah, it'd be the middle of the day,
Right way up I cannot plainly say,
Here's the key, let's end the fun,
I'll tell you a secret, I hide behind one
What am I?