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Neusis trisection of an angle θ>135° to find φ=θ/3, using only the length of the ruler. The radius of the arc is equal to the length of the ruler. Used in en:Neusis_construction.

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u=2cm;
path unitcircle;   unitcircle=fullcircle scaled 2u;
path tick[];   tick0:=(-1mm,0)--(1mm,0);   tick1:=tick0 rotated 90;
thick=1mm;
thin=0.25mm;
path ruler[],anglemark[];
beginfig(3)
    pickup  pencircle scaled thin;
    theta=162;
    z0=(0,0);
    z1=(u,0)=-z3=-.5z3l;
%  z2=(0,u)=-z4;
    z5=u*dir(theta);
    z7=(-u,0) rotated (-theta/3);
    y6=y0;
    x6=2x7;
    ruler0:=unitsquare xscaled u yscaled .1u rotated angle(z7-z6) shifted z6;
    ruler1:=unitsquare yscaled 0.5 shifted 0.25up xscaled u yscaled .1u rotated angle(z7-z6) shifted z6;
    fill ruler0 withcolor (1,1,.5);
    fill ruler1 withcolor (.9,.9,.3);
    draw ruler0 withpen pencircle scaled 0.25;
    draw subpath(0,4) of unitcircle ;
    draw z1--.3[z3,z3l] ;
    draw z0--z5 ;
    label.bot("O",z0);
    label.bot("A",z6);
    label.ulft("P",z5);
    label.top("B",z7);
    anglemark0= unitcircle scaled 0.2 cutbefore (z0--z1) cutafter (z0--z5);
    draw anglemark0;
    label.urt(btex $\theta$ etex, point .4length(anglemark0) of anglemark0);
    draw z6--z7--z0 withcolor blue;
    anglemark1= unitcircle scaled 0.3 cutbefore (z0--z7) cutafter (z0--z3);
    label.ulft(btex $\varphi$ etex, point 0 of anglemark1 rotated (theta/6)) withcolor blue;
    draw anglemark1 withcolor blue;
    for i = 0,5,6,7:
        if known(x[i]):
            drawdot z[i] withpen pencircle scaled thick;
%          label(decimal i,z[i]) withcolor green;
        fi
    endfor
    clip currentpicture to bbox( (.4u,0)--(0,u)--(-1.25u,0)--(0,-0.25u));
endfig;
bye