typedef struct 
{
  float x;
  float y;
} point;

typedef struct
{
  point a;
  point b;
} line;

int main(void)
{
  line m;

  m.a.x = 1.2;
  m.a.y = 7.6;
  m.b.x = 38.5;
  m.b.y = 17.8;
  ...