These are examples of iambic tetrameter, the basic template being: * / | * / | * / | * /

                            I have no wit, no words, no tears;
                            My heart within me like a stone
                            Is numbed too much for hopes or fears.
                            Look right, look left, I dwell alone;
                            I lift mine eyes, but dimmed with grief
                            No everlasting hills I see;
                            My life is in the falling leaf:
                                                    --Christina Rossetti

                    SWEET, be not proud of those two eyes
                    Which starlike sparkle in their skies;
                    Nor be you proud that you can see
                    All hearts your captives, yours yet free;
                    Be you not proud of that rich hair
                    Which wantons with the love-sick air;
                    Whenas that ruby which you wear,
                    Sunk from the tip of your soft ear,
                    Will last to be a precious stone
                    When all your world of beauty's gone.
                                                --Robert Herrick

In William Blake's poem below, the dominant meter is iambic tetrameter, but a number of the lines lack their initial syllable (*), making them headless. This is an acceptable variation, though sometimes it does make the meter somewhat ambiguous.

                                 I saw a chapel all of gold
                                 That none did dare to enter in,
                                 And many weeping stood without,
                         (*) Weeping, mourning, worshipping.
                                 I saw a serpent rise between
                         (*) The white pillars of the door,
                         (*) And he forc'd and forc'd and forc'd,
                         (*) Down the golden hinges tore.

Tennyson's "The Owl" includes some interesting substitutions, including one that almost makes us think he has changed from tetrameter to trimeter (the lines beginning "And the . . ."). We can consider these as pyrrhic substitutions--one of which is also an interesting case of a pyrrhic followed by a trochee ("whirring")--a rare, but evidently possible, combination, since he did it.

                           When cats run home and light is come,
                                 And dew is cold upon the ground,
                            And the far-off stream is dumb,
                                 And the whirring sail goes round,
                                 And the whirring sail goes round;
                                      Alone and warming his five wits,
                                      The white owl in the belfry sits.

                            When merry milkmaids click the latch,
                                 And rarely smells the new-mown hay,
                            And the cock hath sung beneath the thatch
                                 Twice or thrice his roundelay,
                                 Twice or thrice his roundelay;
                                      Alone and warming his five wits,
                                      The white owl in the belfry sits.