<p>Simple Celtic key pattern inside an overall Latin Cross shape.  This is partially loosely based on a design in "Celtic Art: The Methods of Construction" by <a href="https://en.wikipedia.org/wiki/George_Bain_(artist)" class="extiw" title="en:George Bain (artist)">George Bain</a>, which was in turn loosely based on patterns in the Book of Kells.
</p>
If the lighter green path were to be cut somewhere on the outside of the cross, it would become a labyrinth which returns to its starting point.
AnonMoos
<p>Self-made image, generated from the following vector PostScript source code:<br clear="all"></p>
<pre>%!
306 484 translate 2.4 dup scale/A 1800 sqrt 1 add def/B 1800 sqrt 5 sub
dup mul dup add sqrt def/C B 5 sub def/F B 15 sub def/D F 2 div def/E B
2 sqrt div 2 mul def 0 .1667 0 setrgbcolor A neg A moveto
A neg A E add lineto A A E add lineto A A lineto A E add A lineto
A E add A neg lineto A A neg lineto A A neg E 2 mul sub lineto
A neg A neg E 2 mul sub lineto A neg A neg lineto
A neg E sub A neg lineto A neg E sub A lineto closepath fill
8 setlinewidth 0 .6667 0.1111 setrgbcolor gsave 0 E translate 45 rotate
5 C neg moveto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto
5 D lineto 15 D lineto 15 5 lineto C 5 lineto 0 B lineto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 F neg lineto -15 F neg lineto
-15 -5 lineto B neg -5 lineto stroke grestore gsave E neg 0 translate
45 rotate B -5 moveto 15 -5 lineto 15 F 10 sub neg lineto
F 10 sub F 10 sub neg lineto F 10 sub F neg lineto 5 F neg lineto
5 D lineto 15 D lineto 15 5 lineto C 5 lineto 0 B lineto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 D neg lineto -15 D neg lineto
-15 -5 lineto C neg -5 lineto -5 C neg lineto stroke grestore gsave
45 rotate B 5 add neg 5 moveto -15 5 lineto -15 F lineto -5 F lineto
-5 F neg lineto F 10 sub neg F neg lineto
F 10 sub neg F 10 sub neg lineto -15 F 10 sub neg lineto -15 -5 lineto
B 5 add neg -5 lineto stroke B 5 add -5 moveto 15 -5 lineto
15 F neg lineto 5 F neg lineto 5 F lineto F 10 sub F lineto
F 10 sub F 10 sub lineto 15 F 10 sub lineto 15 5 lineto
B 5 add 5 lineto stroke grestore gsave E 0 translate 45 rotate
B neg 5 moveto -15 5 lineto -15 F 10 sub lineto
F 10 sub neg F 10 sub lineto F 10 sub neg F lineto -5 F lineto
-5 D neg lineto -15 D neg lineto -15 -5 lineto C neg -5 lineto
0 B neg lineto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto
5 D lineto 15 D lineto 15 5 lineto C 5 lineto 5 C lineto stroke
grestore gsave 0 E neg translate 45 rotate -5 C moveto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 F neg lineto
F 10 sub neg F neg lineto F 10 sub neg F 10 sub neg lineto
-15 F 10 sub neg lineto -15 -5 lineto B 5 add neg -5 lineto stroke
5 C neg moveto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto
5 F lineto 15 F lineto 15 5 lineto B 5 lineto stroke grestore gsave
0 E 2 mul neg translate 45 rotate -5 C moveto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 D neg lineto -15 D neg lineto
-15 -5 lineto C neg -5 lineto 0 B neg lineto C -5 lineto 15 -5 lineto
15 D neg lineto 5 D neg lineto 5 F lineto 15 F lineto 15 5 lineto
B 5 add 5 lineto stroke grestore 2 setlinewidth 0 .1667 0 setrgbcolor
A 1 sub neg A 1 sub moveto 5 -5 rlineto stroke
A 1 sub A 1 sub neg moveto -5 5 rlineto stroke showpage
%EOF</pre>
<p>The following is the vector PostScript source code for the basic square, six of which are used to form the cross:
</p>
<pre>%!
300 396 translate 4 dup scale/A 1800 sqrt 1 add def/B 1800 sqrt 5 sub
dup mul dup add sqrt def/C B 5 sub def/D B 15 sub 2 div def A A moveto
A neg A lineto A neg A neg lineto A A neg lineto closepath fill gsave
45 rotate 8 setlinewidth 1 setgray -15 5 moveto -15 D lineto -5 D
lineto -5 D neg lineto -15 D neg lineto -15 -5 lineto C neg -5 lineto 0
B neg lineto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto 5
D lineto 15 D lineto 15 5 lineto C 5 lineto 0 B lineto C neg 5 lineto
closepath gsave stroke grestore showpage
%EOF</pre>