When we talk about walking, we can say you go on foot or by foot, as a mode of transport. Which preposition is correct? Technically, on is more accurate, and common, and in exams you may be marked incorrect for using by foot. But why is by foot a mistake? Or is it a mistake at all? This is a perfect example of English grammar as a matter of style, not accuracy – and as you’ll see here, both are actually possible.
We can use say both – with these justifications:
Why you should say on foot
We usually use on for movements or actions that involve using body parts. You can rest on your elbows, you can pray on your knees, and you can lie on your back. Walking is no different – the action takes place on foot.
Why you could say by foot
We use by to describe a tool used for movements or actions. You can travel by plane, by boat or by car, for instance. You can send letters by post, and you can write by hand. It stands to reason, then, that you can travel by foot.
When you need to decide on proper preposition use, there are sometimes two possibilities which apparently have the same meaning, though one may be more common and seen as correct. This is particularly true when there is some crossover between the use of the preposition to link nouns. In this case, on can be used to mean the object, surface or means we travel atop, while by can mean the method transport we use. Travelling on something and using something to travel (by) can therefore produce the same meaning, with a different grammatical construction.
So which is more correct?
You move with your feet in contact with the ground, supported by your feet, making on foot a more literal description of the action. Yet your feet are also a tool of sorts, so if we want to describe what you use to travel, by foot makes perfect sense. It is not reasonable, then, to say one is more correct than the other. Yet you may be told that by foot is incorrect. Why? For the same reason that many rules exist in English:
Because on foot is more commonly used than by foot.
The English language has adapted over many centuries, and different rules come and go when it comes to grammar. It is important to recognise when a rule emerges as a matter of style, or a matter of meaning, however. Many grammar rules exist to help clarify what is said. In this case, there is no firm rule, because one expression is not more clear than the other.
You may find English people argue about this, and many other rules and sayings, because one style sounds strange to the person who uses the other. These arguments serve no grammatical purpose, and work only to limit the language. When it comes to matters of style, choose the option that you prefer. You will not be misunderstood.