Stress and intonation
Consonants and vowels are called SEGMENTS because they follow from a segmentation of a stretch of speech into discrete phonetic slices. But there is more to the speech signal than just these segments. By vibrating the vocal cords faster or slower, one changes the pitch of a voiced sound, respectively raising or lowering it. By making the vibration more forceful and energetic, one increases its loudness (volume, intensity). These variations in pitch and loudness are termed SUPRASEGMENTALS since they are perceived in addition to, and organized "on top of", the segments. They are also called by their classical name, PROSODIES.
A vowel or the syllable around it that is pronounced more loudly than its neighbors is STRESSED; one that is not especially loud is UNSTRESSED. Stress is indicated by an accent mark on the vowel or (the IPA's preference) in front of the syllable: rejéct, re1ject. Unstressed vowels are left unmarked in transcription, although a breve (˘) can be used over them if necessary to draw attention to their lack of stress.
English and Spanish make similar use of stress, as noted in the following section, but there are two differences in its phonetic realization. First, Spanish stress equates to greater loudness, whereas English speakers often reinforce loudness with higher pitch. Carried over into Spanish, this coupling of stress with pitch can disrupt Spanish intonational patterns, which are generally flatter than those of English (v. 4.2). Second, stressed and unstressed vowels in Spanish do not significantly differ in length (quantity) or articulation (quality); lengthening is exceptional and signals strong emphasis (Bull 1965, 78; Navarro Tomás 1967, 200–207). In English, however, stressed vowels are normally lengthened (V:) while unstressed syllables are correspondingly shortened and reduced to schwa (v. 3.2.7). Transferred to Spanish, this stretching/shortening effect makes paró, pare sound like p'róóóu, p'réééi and is a well-known trait of many English and German speakers' pronunciation of Spanish (Navarro Tomás 1967).