Stress refers to the degree of force used in producing a syllable. A basic distinction is made between stressed and unstressed syllables, the former being more prominent than the latter, which means that stress is a relative notion. At the word level, it only applies to words with at least two syllables. At the sentence level, a monosyllabic word may be said to be stressed relative to other words in the sentence. Stress may change over history and exhibit regional or dialectal differences, and sometimes it is placed on a different syllable for the different grammatical functions a word plays. For example, the form “produce” is pronounced different and functions differently when stress is put on the first and the second syllable respectively. Sentence stress is more interesting. In general situations, notional words are normally stressed while structural words are generally unstressed.