Classical Guitar Barre 1

Barre     B1, B2, B3, B4, ……

Classical Guitar Barre 2

 (French) A device that clamps to the neck of a plucked string instrument
and which change its tuning by shortening the sounding length of every string.
A single finger holding multiple strings on a stringed instrument  at the same time
Symbol used in standard notation for guitar. Example: B7 (C7, CVII, VII)
A dashed bracket    following a Barre symbol indicates duration.

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Two Bars at Once

Classical Guitar Barre 3

In this example the 4th finger Bars the 5th fret
while the 2nd finger bars the second fret.

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Anticipated or Expected Barre

Classical Guitar Barre 4

1st finger is fretted (at lower F note) then tilted at measured 2nd half step
into  B1 (Barre 1) to position for upper F note in one flowing motion as
lower note rings through the duration.