chord progression

[quaver]

Can someone explain why the V6/4 is not called I 6/4. in the I IV V6/4-5/3 progression. After all it has the notes of the tonic triad and not the notes of the dominant triad. Hopefully I have a snapshot page for you to see but Im not sure how to attach it.

[nystul]

When this chord resolves to a dominant harmony, it is called the cadential 6/4, and it is considered part of the dominant function.  That is why there is disagreement on how to label it.  Some texts do call it I 6/4.

[faulty_damper]

Yes, some texts refer to it as a I 6/4.  Regardless of how it is labeled, what is important is how it sounds, the musical expression of the chord.  Labels make no sound.