Sub vs infra

As a prefix, sub- means that the modified word is underneath, below, or in some way less than something else. It can mean a lower level of classification or a smaller part of something bigger.

With one exception, sub- words do not use a hyphen inside the United States. Outside, including Canada, most sub- words use a hyphen in their official spellings.

Infra- is also an prefix that can mean below, but usually means inside or within.


With a few exceptions, whether the classification uses sub- or infra- seems to be largely a matter of preference for the scientist naming the category. One could assume (taking into account the risks in doing so) that perhaps in the distinct examples one word already existed when another was needed.

One example of a distinct difference is subsonic and infrasonic. Subsonic is an adjective describing things as moving slower than sound moves. It dates from the 1930s. Infrasonic is an adjective describing things as being related to a certain range of sound that humans can’t hear. It dates from the 1920s. In this example, infra- means below on a scale and sub- means less in a comparison.

Another example is infrastructure and substructureInfrastructure is the basic needs for a civilization to function, also the framework or foundation of an organization or group. This term dates to the 1930s. Substructure is simply a supporting part of a building that is underneath something else. This term dates to the 1700s. In this example, infra- means within or inside and sub- means below.


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  1. GoatGuy says:

    Well… that was interesting! I went on a hunt for “all words that contain (sub | under | infra) and share a tail-stem. Words like (substructure | understructure). ( the vertical bar in many branches of computational linguistics means ”or“)

    There are a LOT of (sub | under) matches.

    substrata, understrata. subtone, undertone. subworld, underworld.subsoil, undersoil.subsecretary, undersecretary.subplot, underplot.sublet, underlet.sublease, underlease.subframe, underframe.

    And so on. But there are very few that have (sub | infra) commonality! Remarkable, actually:

    subclass, infraclass, underclass. (+ plurals, etc.)subhuman, infrahumansubstructure, infrastructure, understructure

    And that’s it as far as I can glean. What is pretty clear is that sub- and under- are not functional synonyms as prefixes go. sub- seems to fill the role of being a part of, whereas under- is relegated to being less than, or, being lower than, or beneath.

    More than this I cannot say.


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