How to Scan a Line of Blank Verse

1. Start with the assumption that in all likelihood the line will run as follows: ˘´ | ˘´ | ˘´ | ˘´ | ˘´ |. If there are deviations from this pattern, they will probably be few and will probably be in the early part of the line so that the normal rhythm can reassert itself before the end. Be prepared (note #4 below) to listen to the rhythm that your proposed scansion produces and to decide whether that rhythm could be what the poet would have wanted. Scansion determines which syllables count as beats in the rhythm.

2. Divide the line into pairs of syllables. Do this before, not after, deciding where the beats are, for this is one of the methods of making that determination. This is the stage at which your task most radically differs from the discovery of rhythm in prose. In prose a reader has nothing to go on except the rules of word stress (as shown in the dictionary) and rather flexible rules of phrase stress. But the counting of syllables in verse provides guidance that readers of prose do not have.
      There are to be five feet in the line, and there can be no more than eleven syllables (the eleventh, if any—always weak and at the end of a line—is called an "extrametrical syllable"). In counting the syllables, be aware that poets often elide adjacent vowels into one syllable (cru-el==>crool) or even drop syllables entirely, as we do in making contractions. They may also (though rarely) diphthongize a vowel so as to stretch it into two syllables (fi-re). Spenser, for instance, uses two pronunciations for "puissance" (meaning power or strength): /pwi-sance/ and /pu-wi-sance/—depending on his metrical needs.

3. Determine the strength of the syllables in the line.The decisions you make will yield one and only one beat per foot, for the line has five and only five beats. Where these beats fall depends not only on the natural stresses of the words and phrases but also on the counting of syllables, which adds one extra stage of comparing weights, forcing you to discriminate between adjacent syllables that ordinarily might not be compared or might be felt to be equal. However, the normal ways of comparing them (within a word, within a phrase) still operate. Thus, in "You sul|f'rous and | thought-ex|ecut|ing fires|" you would not ordinarily compare the end of "sulf'rous" with "and" because they belong to different phrases, but the foot divisions force you to do so. On the other hand, you can't ignore the normal comparisons within the word, which force "-r'ous" to be heard as weak.
      Be sure that you do not violate the stress pattern of any word as shown in the dictionary. Your scansion must sound like human speech when you are done with it.
When I score the scansion, I will take into account that placing a stress mark over every even-numbered syllable takes no brain-power to speak of and that even though 98% of the feet will be stressed in this manner, little credit is due to a student who misses the other 2%.  To put the matter more positively, it takes keen attention and acuteness of ear to recognize the one foot in fifty which happens to be inverted (| ´ ˘ | instead of | ˘ ´ |).  That acuteness, that attention—and the accuracy of the foot divisions—are what I will be grading.

4. Check the rhythm you have created. Tap it with your finger or sound it out with "ti tum ti tum." Is it fluent or stumbling? If the latter, could you have made a mistake?
If you have marked two adjacent syllables as strong, they must be in different words as well as in different feet. Rhythm requires a certain degree of regularity in the time between the beats, or it ceases to be discernible as rhythm and becomes noise. Word division is the least amount of space permissible between two beats. (This maxim has been demonstrated by statistical analysis.) If you have two adjacent feet marked as inversions (trochees: ´ ˘ | ´ ˘), check to see if you have made a mistake. This almost never happens, for the rhythm it creates is very awkward. Still more unlikely is an inverted fifth foot. Shakespeare wrote only one line in his entire corpus with an inverted last foot: the mad and dying Lear says, "Never, never, never, never, never." Make "never" your watchword for inverted fifth feet!