what does this symbol mean?

Logic gate symbols

The AND, OR, NOT, NAND, NOR, XOR and XNOR gate symbols in both standard shapes, with each gate’s truth table and Boolean notation.

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About logic gate symbols

A logic gate takes one or two inputs, each 0 or 1 (low or high), and gives one output by a fixed rule. Each gate has a symbol, a truth table listing the output for every combination of inputs, and a way of writing it in Boolean algebra. Three gates are basic: AND, whose output is 1 only when all inputs are 1; OR, 1 when any input is 1; and NOT, the inverter, which outputs the opposite of its input.

The inverted gates

A small circle on a gate, the inversion bubble, flips its output. With it AND becomes NAND and OR becomes NOR. Kuphaldt’s textbook Lessons in Electric Circuits calls NAND and NOR universal: given enough of either, any other gate can be built from them. Without the circle, the inverter’s triangle is a buffer, which passes its input through unchanged and only strengthens it.

Exclusive OR and NOR

XOR, exclusive OR, outputs 1 when its inputs differ, and its symbol is the OR shape with a second curved line behind it. XNOR adds the circle and outputs 1 when the inputs are the same. Boolean algebra writes XOR as A ⊕ B, as Texas Instruments’ data sheets do.

Two ways to draw each gate

Two standards draw gates. The distinctive shapes, the D of AND and the pointed OR, are the ones in ANSI/IEEE Std 91, and the drawings on these pages use them. The IEC system, IEC 60617-12, draws every gate as a rectangle and puts a qualifying symbol inside: & for AND, ≥1 for OR, =1 for exclusive OR and 1 for a buffer, with a negation circle where the gate inverts. Texas Instruments’ guide to IEEE Std 91 notes that in this system the shape of the outline is not significant. Each page sets both forms side by side.

Sources

  1. Lessons In Electric Circuits, Volume IV: Digital, chapter 3, Logic Gates, Tony R. Kuphaldt, open textbook (Design Science License), Tony R. Kuphaldt (ibiblio), Logic gates
  2. Overview of IEEE Standard 91-1984: Explanation of Logic Symbols, SDYZ001A, 1996, Texas Instruments, §1 Introduction; §2; Table 1
  3. SNx4HC86 Quadruple 2-Input XOR Gates (data sheet), SN74HC86 data sheet, Texas Instruments, 3 Description

Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research