Usage: Handing over the 'reigns'

Stumbling over the "rein/reign" usage difference is common as in this sentence this morning from Mediabistro's All Twitter in a story about Twitter co-founder Biz Stone leaving the company:

Now, it seems, Stone is ready to hand over the reigns and begin working on other projects.

It should be hand over the reins, not reigns. It flows from the same idea as "free rein," and "rein in," not "reign."

Whenever you come across these, remember, they all come from the days when horses were common and reins were the leather straps you used to control them: You reined in the horse (or gave it free rein), handed over the reins, etc.

Nothing to do with royalty here, unless you tend to equate royalty with the working end of a horse ...