When you use GpiPolySpline to draw a spline, each curve is tangential to the first and last of three connected lines. The following figure shows how to draw a spline.

#include <os2.h>
    HPS hps;                        /* Presentation-space handle              */

MRESULT EXPENTRY wpClient(HWND hwnd, ULONG msg, MPARAM mp1, MPARAM mp2){
        POINTL aptl[3];             /* Structure for control points           */

        aptl[0].x = 50;             /* Loads x-coord. of first control point  */
        aptl[0].y = 100;            /* Loads y-coord. of first control point  */
        GpiMove(hps, aptl);         /* Sets current position                  */
        aptl[0].x = 75;             /* Loads x-coord. of second control point */
        aptl[0].y = 200;            /* Loads y-coord. of second control point */
        aptl[1].x = 100;            /* Loads x-coord. of third control point  */
        aptl[1].y = 0;              /* Loads y-coord. of third control point  */
        aptl[2].x = 125;            /* Loads x-coord. of fourth control point */
        aptl[2].y = 100;            /* Loads y-coord. of fourth control point */
        GpiPolySpline(hps, 3L, aptl);   /* Draws spline                       */
} /* wpClient */