As a follow-up to my last Tip of the Day, let’s talk about the difference between Onto and On To. Although similar to the Into vs. In To discussion, this one is a little more cut and dried, black and white, so you don’t have to say “That depends.”
Verb phrases that include the word On (hold on, come on, go on) behave similarly to verb phrases that include the word In when they encounter the preposition To; i.e., they avoid each other.
Cowboy Bill, you better hold on to the reins a bit tighter when you’re riding Ol’ Hurricane!
Cowboy Bill was coming on to the pretty lady at the Three Springs Saloon, but he decided to go on to the bunkhouse when she showed him her wedding ring.
The Chicago Manual of Style has a nifty way to tell when to use Onto:
If you can insert the word Up in front of the On To and it makes sense, use
The cat jumped (up) onto the kitchen counter and started eating the tuna salad she had just prepared for sandwiches.