Morphology and Word Formation › Compounding

Compounding combines two or more free morphemes into a single new word: toothbrush, bookstore, laptop, database administrator. Compounding is extraordinarily productive in English, German (Donaudampfschifffahrtsgesellschaft, "Danube steamship shipping company"), and many other languages, and it generates open-ended novelty that no fixed lexicon can capture. Unlike affixation, compounding draws its raw material entirely from existing free words, which means the space of possible compounds grows at least quadratically with the vocabulary itself (faster still for multi-part compounds, and unboundedly under recursion).

Three properties matter for NLP:

• Headedness: compounds have a head that determines the category and core meaning. A doghouse is a kind of house, not a kind of dog. The head is typically rightmost in English and leftmost in Romance languages. Semantic interpretation therefore depends on identifying the head, a task related to parsing that we revisit in the syntax chapters. Get the head wrong and you invert the meaning: treating doghouse as a kind of dog produces nonsense that no amount of embedding smoothing will repair.
• Recursion: compounds nest. Language technology research funding is a compound built from smaller compounds, and its bracketing determines its meaning. Ambiguity is real: a Spanish teacher could be a teacher of Spanish or a teacher who is Spanish. Just as bracketing determined the meaning of unhappiness earlier, bracketing inside compounds determines what modifies what, and there is often no local cue that resolves it.
• Orthographic instability: English writes compounds inconsistently as closed (notebook), hyphenated (mother-in-law), or open (ice cream). Whether hot dog is one word or two depends on whether lunch or dogs is under discussion. This variability directly complicates the word tokenization we studied in Part I, since a whitespace-based tokenizer silently splits open compounds and merges nothing. Worse, the same compound can migrate between styles over time or across style guides, so even a curated list of known compounds goes stale.