==Definition==
[[Sandhedstabellen]] for ''P'' <math>\Leftrightarrow</math> ''Q'' er som følger - sidste søjle (S=sand, F=falsk):<ref>[http://www.wolframalpha.com/input/?i=p+%3C%3D%3E+q p <=> q]. [[Wolfram Alpha|Wolfram&#124;Alpha]]</ref><ref>{{Cite web|title=If and only if |publisher=UHM Department of Mathematics |url=http://www.math.hawaii.edu/~ramsey/Logic/Iff.html |archive-url=https://web.archive.org/web/20220407061628/https://math.hawaii.edu/~ramsey/Logic/Iff.html |archive-date=2022-04-07|quote=Theorems which have the form "P if and only Q" are much prized in mathematics. They give what are called "necessary and sufficient" conditions, and give completely equivalent and hopefully interesting new ways to say exactly the same thing.}}</ref>
{| class="wikitable" style="margin:1em auto; text-align:center; float:left"
|+ Sandhedstabel
|-
! scope="col"  style="width:20%" | ''P''
! scope="col"  style="width:20%" | ''Q''
! scope="col"  style="width:20%" | {{nowrap|''P''&nbsp;<math>\Leftrightarrow</math> ''Q''}}
|-
| S || S  || S
|-
| S || style="background:papayawhip" | F || style="background:papayawhip" | F
|-
| style="background:papayawhip" | F || S || style="background:papayawhip" | F
|-
| style="background:papayawhip" | F || style="background:papayawhip" | F || S
|}
{{clear}}
Filosofi Videnskab Matematik Naturvidenskab