<p>This <a href="http://www.allaboutcircuits.com/vol_4/chpt_9/1.html" rel="nofollow">source</a> states that it comes from the concept of a <a href="http://en.wikipedia.org/wiki/Combination" rel="nofollow">combination set</a>, which is backed up by Hennesy &amp; Patterson's explanation in "Computer Organization and Design", p.340.</p>

<p>The reasoning behind it seems to be as follows. In a combination the order of elements is irrelevant: in (<em>n</em> choose <em>k</em>) it doesn't matter in which order you pick the <em>k</em> elements. Similarly, a combinational logic circuit is a logic circuit where the order of inputs doesn't matter for the outputs. Note here that "an input" refers to the <em>entire</em> input to the circuit, not the individual lines. So, the intuition is that you give a combinational circuit some sequence of inputs, and "order doesn't matter" is true in the sense that the output for each input is the same, even if you permute the sequence.</p>

<p>This is to be understood in opposition to <em>sequential</em> logic, where the order of inputs <em>does</em> matter, because sequential circuit can have internal state (e.g. flip-flops) that can and usually influence individual outputs.</p>

CC BY-SA 3.0
2012-11-23T09:14:35.217Z
2012-11-23T09:14:35.217Z
funkstar
<p>What is the significance of the word "combinational" in <a href="http://en.wikipedia.org/wiki/Combinational_logic" rel="nofollow">combinational logic</a>?</p>

<lo.logic><terminology>
Why is combinational logic called so?
Rob Jensen
# Why is combinational logic called so?

(Asked by: username_0 on 2012-11-22)

What is the significance of the word "combinational" in [combinational logic](http://en.wikipedia.org/wiki/Combinational_logic)?

**Tags:** lo.logic, terminology


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## 1 answers

### Answer by username_1 on 2012-11-23. Score: 2 | Accepted answer

This [source](http://www.allaboutcircuits.com/vol_4/chpt_9/1.html) states that it comes from the concept of a [combination set](http://en.wikipedia.org/wiki/Combination), which is backed up by Hennesy & Patterson's explanation in "Computer Organization and Design", p.340.

The reasoning behind it seems to be as follows. In a combination the order of elements is irrelevant: in (*n* choose *k*) it doesn't matter in which order you pick the *k* elements. Similarly, a combinational logic circuit is a logic circuit where the order of inputs doesn't matter for the outputs. Note here that "an input" refers to the *entire* input to the circuit, not the individual lines. So, the intuition is that you give a combinational circuit some sequence of inputs, and "order doesn't matter" is true in the sense that the output for each input is the same, even if you permute the sequence.

This is to be understood in opposition to *sequential* logic, where the order of inputs *does* matter, because sequential circuit can have internal state (e.g. flip-flops) that can and usually influence individual outputs.


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