Directionality of word class conversion

Many nouns (like google, brick, bike) can be used as verbs:

  • Let me google that for you.
  • The software update bricked my phone.
  • Bob biked to work yesterday.

Conversely, many verbs (like talk, call) can be used as nouns:

  • She gave a talk at the conference.
  • I’m on a call with my boss.

Here, we just assumed that {google, brick, bike} are primarily nouns and {talk, call} are primarily verbs — but is this justified? After all, all five of these words can be used as either a noun or a verb. Then, what’s the difference between the first group {google, brick, bike} and the second group {talk, call}?

These are examples of word class flexibility: words that can be used across multiple part-of-speech classes. In this blog post, I’ll describe some objective criteria to determine if a random word like “sleep” is primarily a noun or a verb.

Five criteria for deciding directionality

Linguists have studied the problem of deciding what is the base / dominant part-of-speech category (equivalently, deciding the directionality of conversion). Five methods are commonly listed in the literature: frequency of occurrence, attestation date, semantic range, semantic dependency, and semantic pattern (Balteiro, 2007; Bram, 2011).

  1. Frequency of occurrence: a word is noun-dominant if it occurs more often as a noun than a verb. This is the easiest to compute since all you need is a POS-tagged corpus. The issue is the direction now depends on which corpus you use, and there can be big differences between genres.
  2. Attestation date: a word is noun-dominant if it was used first as a noun and only later as a verb. This works for newer words, Google (the company) existed for a while before anyone started “googling” things. But we run into problems with older words, and the direction then depends on the precise dating of Middle English manuscripts. If the word is from Proto-Germanic / Proto-Indo-European then finding the attestation date becomes impossible. This method is also philosophically questionable because you shouldn’t need to know the history of a language to describe its current form.
  3. Semantic range: if a dictionary lists more noun meanings than verb meanings for a word, then it’s noun-dominant. This is not so reliable because different dictionaries disagree on how many senses to include, and how different must two senses be in order to have separate entries. Also, some meanings are rare or domain specific (eg: “call option” in finance) and it doesn’t seem right to count them equally.
  4. Semantic dependency: if the definition of the verb meaning refers to the noun meaning, then the word is noun-dominant. For example, “to bottle” means “to put something into a bottle”. This criterion is not always clear to decide, sometimes you can define it either way, or have neither definition refer to the other.
  5. Semantic pattern: a word is noun-dominant if it refers to an entity / object, and verb-dominant if refers to an action. A bike is something that you can touch and feel; a walk is not. Haspelmath (2012) encourages distinguishing {entity, action, property} rather than {noun, verb, adjective}. However, it’s hard to determine without subjective judgement (especially for abstract nouns like “test” or “work”), whether the entity or action sense is more primary.

Comparisons using corpus methods

How do we make sense of all these competing criteria? To answer this question, Balteiro (2007) compare 231 pairs of flexible noun/verb pairs and rated them all according to the five criteria I listed above, as well as a few more that I didn’t include. Later, Bram (2011) surveyed a larger set of 2048 pairs.

The details are quite messy, because applying the criteria are not so straightforward. For example, polysemy: the word “call” has more than 40 definitions in the OED, and some of them are obsolete, so which one do you use for attestation date? How do you deal with homonyms like “bank” that have two unrelated meanings? With hundreds of pages of painstaking analysis, the researchers came to a judgement for each word. Then, they measured the agreement between each pair of criteria:

bram-thesis-tableTable of pairwise agreement (adapted from Table 5.2 of Bram’s thesis)

There is only a moderate level of agreement between the different criteria, on average about 65% — better than random, but not too convincing either. Only frequency and attestation date agree more than 80% of the time. Only a small minority of words have all of the criteria agree.

Theoretical ramifications

This puts us in a dilemma: how do we make sense of these results? What’s the direction of conversion if these criteria don’t agree? Are some of the criteria better than others, perhaps take a majority vote? Is it even possible to determine a direction at all?

Linguists have disagreed for decades over what to do with this situation. Van Lier and Rijkhoff (2013) gives a survey of the various views. Some linguists maintain that flexible words must be either noun-dominant or verb-dominant, and is converted to the other category. Other linguists note the disagreements between criteria and propose instead that words are underspecified. Just like a stem cell that can morph into a skin or lung cell as needed, a word like “sleep” is neither a noun or verb, but a pre-categorical form that can morph into either a noun or verb depending on context.

Can we really determine the dominant category of a conversion pair? It seems doubtful that this issue will ever be resolved. Presently, none of the theories make any scientific predictions that can be tested and falsified. Until then, the theories co-exist as different ways to view and analyze the same data.

The idea of a “dominant” category doesn’t exist in nature, it is merely an artificial construct to help explain the data. In mathematics, it’s nonsensical to ask if imaginary numbers really “exist”. Nobody has seen an imaginary number, but mathematicians use them because they’re good for describing a lot of things. Likewise, it doesn’t make sense to ask if flexible words really have a dominant category. We can only ask whether a theory that assumes the existence of a dominant category is simpler than a theory that does not.


  1. Balteiro, Isabel. The directionality of conversion in English: A dia-synchronic study. Vol. 59. Peter Lang, 2007.
  2. Bram, Barli. “Major total conversion in English: The question of directionality.” (2011). PhD Thesis.
  3. Haspelmath, Martin. “How to compare major word-classes across the world’s languages.” Theories of everything: In honor of Edward Keenan 17 (2012): 109-130.
  4. Van Lier, Eva, and Jan Rijkhoff. “Flexible word classes in linguistic typology and grammatical theory.” Flexible word classes: a typological study of underspecified parts-of-speech (2013): 1-30.