Laying down her dagger (4.3.22-24)

[This stage direction doesn't appear in the First Folio of 1623, but was added in later editions of the play.]

When Juliet finds Romeo dead in her tomb, she uses his dagger to kills herself.

JULIET. ...O happy Dagger.

'Tis in thy sheath, there rust and let me die

Kills herself. [First Folio, 5.2]

There's no mention of Romeo's dagger in the First Folio—which was also added in later editions of the play—but it seems reasonable to assume that Juliet uses Romeo's dagger to kill herself.

Some analysts assert that Juliet has a dagger with her in the tomb in case she awakes too early from the sleeping potion and she's trapped in the tomb, or if Romeo fails to return to her.

This seems unlikely. In act 4, scene 5, when Juliet is discovered by her Nurse, Lord and Lady Capulet, Friar Laurence, and Paris after taking the sleeping potion, no one makes any mention of a dagger which is lying in plain sight next to Juliet, and it's doubtful whether this or any other dagger is put into the tomb with her.

As for the symbolism of these daggers—whether the same dagger each time it appears or different daggers—neither Juliet nor anyone else makes any mention of the daggers as anything more than what they are, or their intended use.

On two occasions, Juliet intends to use a dagger to kill herself, and on one occasion she actually uses a dagger to kill herself. The daggers simply serve their purpose, and aside from their representation as instruments of violence and death, carry no extrinsic or symbolic meaning.

Due to its phallic shape and size, the dagger in Romeo and Juliet operates as a traditional symbol of masculinity (or more specifically, of male sexuality). Juliet threatens to kill herself with a dagger several times in the play (although she only acts upon this threat only once).

The first time is shortly after learning that she is being forced into a marriage with Paris. Juliet flees to Friar Laurence's cell under the pretense of going to confession; there she tells Friar Laurence that "with this knife" she will kill herself if Laurence does not provide her with a solution to her predicament. This moment foreshadows Juliet's ultimate end.

After creating a plan to fake her own death, Juliet returns to her chambers and lays down her dagger, commanding it to, "lie thou there." This, however, will not be the last time she touches such a weapon. When Juliet awakens from her fake death, she discovers that Romeo has killed himself with poison. Hearing someone approaching in the crypt, Juliet grabs Romeo's dagger and stabs herself:

Yea, noise? then I'll be brief. O happy dagger!
This is thy sheath;
there rust, and let me die.

There is a long connection between death and sex. In fact, the French phrase for an orgasm is "la petite mort," which translates to "the little death." Sex traditionally is used to give life, while death takes it away. Thus, all these instances with the dagger are so symbolic and poignant because it is her love for Romeo and the consummation of their marriage that ultimately leads to Juliet's premature death. In each major crisis, she is near a symbol of his presence in her life--a presence which has largely been sexual. Romeo is the man who takes her virginity... as well as (in an indirect way) her life. It is, thus, tremendously appropriate that she kills herself with his weapon. Just as their marriage and sexual attraction bound them together, so, too, will their deaths.

The dagger Juliet threatens to kill herself with makes three appearances; it appears: 1) with Friar Laurence if he cannot think of a way to end her betrothal to Paris; 2) with her alone in her room just before she drinks the vial; and 3) with her in the tomb. In all three appearances the symbolism is pretty much the same, though there are other specific connotations. It is culturally recognized that because the dagger is a mini sword that can easily be concealed, it is a symbol of deceit and corruption ("What are Daggers?," knife-depot.com). Although, we do see other connotations per scene.

It is not insignificant that Juliet draws out the dagger in front of Friar Laurence and threatens her life with it as a remedy to prevent her from marrying Paris. In this scene, Act 4, Scene 1, it is very clear that Juliet is very close to committing one of two sins. If she were to marry Paris, she would be committing a sin by entering a polygamus marriage. Likewise, Friar Laurence would be committing a sin by knowingly joining two people in a polygamus marriage. Hence, not only does the dagger symbolize Juliet's deceitful marriage to Romeo and Friar Laurence's deceitful plan, it also symbolizes both Juliet's and Friar Laurence's corrupt break from their religion.

In Act 4, Scene 3, Juliet picks up the dagger again while worrying about whether or not the potion will work. She asks herself:

What if this mixture do not work at all?
Shall I be married then to-morrow morning?
No, no: this shall forbit it: lie thou there. (21-23)

Since she is willing to use the dagger to prevent her marriage to Paris, the dagger also symbolizes her frailty as a woman. She is trapped by her father's command to marry Paris, and one of her only means of rescuing herself is by taking her life with a mini sword, rather than combating the situation with a real sword, as a man would do.

Finally, Juliet uses the dagger to kill herself in her tomb after she sees Romeo lying there dead. At this point, the dagger represents the fraud she has committed in secretly marrying Romeo and faking her death. But it also represents the treachory associated with the families' feud, which is ultimately to blame for both her faked death and her real death.