Trimeter means a poetic line with three metrical feet. These feet do not have to be iambic; they can be trochaic, anapestic, or dactylic, or other variations. Tennyson here, in “Break, Break, Break”, is varying the foot types in an effort to simulate the sound of ocean waves upon the beach. If we take a line, say “And I would that my tongue could utter” we can find three breaks – And I would', that my tongue', could utter' – and we can name them. In a line like “But O for the touch of a vanished hand,” however, there are four such breaks – But O’, for the touch’, of a van’-, and -ished hand’. So this line is not trimeter. Similarly, the line “But the tender grace of a day that is dead” shows four breaks: “But the ten-,’ -der grace’, of a day’, that is dead’.” These two lines, both third lines in their respective stanzas, deviate from the trimeter structure begun so brilliantly with the title of the poem: Break, break, break. (The third lines of the other stanzas are trimeters.)“Reading” the feet, and therefore the rhythm, of a poem requires much practice and a lot of reading poems out loud. Naming the kinds of feet is a skill of a different order, but the mnemonic device that works best is: “Trochee” is a trochee, but “anapest” is dactylic. And the common foot, the iambic, is common in Shakespeare’s lines -- the ubiquitous iambic pentameter.