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I have seen people using the combination of the 2 prepositions to and by to by in some sentences already where, for me, a simple by or a simple to would be enough. Could you explain the situations where should we use to by, where we could not use simply by or to?

Here's a concrete example taken by a book I a reading on computer architecture, where I would simply use to:

The microinstruction branched to by main loop is the one labeled iadd1.

Even tough it's impossible to describe all the situations where this combination could be used, there might exist some rules or situations where it has to be strictly used, otherwise I will never use it, but I would like to understand better why people use it.

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    This is not a compound preposition. To belongs to branched, by designates the agent. "The main loop branches to the microinstruction labeled iadd1. – StoneyB Dec 31 '14 at 19:01
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To by doesn't actually mean anything. At least, a native speaker doesn’t hear it as one thing.

Here's another way to write your example sentence, which might explain what’s going on:

The main loop branches to the microinstruction labeled iadd1.

Branch to is a phrasal verb. So, in the original example, a listener hears branched to as one idea. Its object is the microinstruction.

By the main loop is a prepositional phrase. It says that the subject of branched to is the main loop. It’s a passive construction, just like “The car driven by Bruce is labeled ‘Batmobile’.” Driven is an ordinary verb. Branch to is a phrasal verb: it consists of more than one word. That probably led to your confusion. (Some more about phrasal constructions in English and the difficulty of getting accustomed to them is here.)

The author chose to put branch to into a passive construction in order to emphasize the microinstruction rather than the main loop.

  • Yes, thanks. So when I see to by I can simply think that to probably belongs to a verb to form a phrasal verb, right? – nbro Dec 31 '14 at 19:06
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    @Rinzler Yes. I never thought about that before, but that sounds like a good idea. Once you’re accustomed to the language, you hear the phrasal verbs as indivisible units, but until you’re accustomed to them, they can be hard to sort out! – Ben Kovitz Dec 31 '14 at 19:14

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