Two torch-towers feeding a shared line — output lights only when both inputs are on. Footprint 3 wide × 5 deep × 2 tall. Build it block by block in Minecraft.
How to build it
The gate is a flat 3-block-wide row with two input torches on the
ends and an output torch on the front of the middle block.
Place a 3-block-wide row of solid blocks where the gate will
live.
Drop a lever on the ground behind each end block, with a short
trail of redstone dust running from the lever to the back of
that block.
Stand a redstone torch on top of each end block — those are the
input inverters: torch is on when the lever is off.
Run redstone dust across the top of the middle block so the two
inverters share one horizontal line.
Mount one more redstone torch on the front face of the
middle block. That's the output torch — it lights only when both
inverters are off, i.e. when both inputs are on.
Run dust forward (or aim straight at a note block) to pick up the
output.
Why it works
It's the classic De Morgan build: A ∧ B = ¬(¬A ∨ ¬B).
Invert A with a torch → ¬A.
Invert B with a torch → ¬B.
If either inverter is on (an input is off), the middle is powered,
and the output torch is off.
The middle output torch lights only when both inverters are
off — i.e., both inputs are on.
Gotchas
The two input torches must not be close enough to cross-power each
other's blocks. The row-of-three layout keeps them separated.
This version has three torch delays, so the gate reacts about
three redstone ticks after the input changes. Compact tile-based
variants exist that are smaller and faster — this one is built to
be readable.