#include <bits/stdc++.h>
using namespace std;

#define REP(i,a,b) for(int i=(a),i##_end_=(b);i<i##_end_;++i)

static const int M=1003;
int T,n,k;
int F[M*M];
vector<int>V[2][3];
int G[2*M*M];
bool Work(int p){
  REP(i,0,V[p][0].size()+1)
    REP(j,0,V[p][1].size()+1){
      //cerr<<i<<','<<j<<','<<k-i*2-j*3<<endl;
      if(((k-i*2-j*3)&3)==0 && (k-i*2-j*3)>=0 && ((k-i*2-j*3)>>2)<=V[p][2].size()){
        REP(a,0,i)F[V[p][0][a]]^=1;
        REP(a,0,j)F[V[p][1][a]]^=1;
        REP(a,0,(k-i*2-j*3)>>2)F[V[p][2][a]]^=1;
        return 1;
      }
    }
  return 0;
}
int main(){
  scanf("%d",&T);
  while(T--){
    scanf("%d%d",&n,&k);
    REP(i,0,2)REP(j,0,3)V[i][j].clear();

    if(n==1){
      if(!k)printf("Possible\nR\n");
      else puts("Impossible");
      continue;
    }
    if(n==3&&k==5){
      printf("Possible\nRBB\nRRB\nRBR\n");
      continue;
    }
    if(n==3&&k==7){
      printf("Possible\nBBB\nRBR\nRRR\n");
      continue;
    }
     if(k==1 || k==2*n*(n-1)-1){
      puts("Impossible");
      continue;
     }

    REP(i,1,n+1)REP(j,1,n+1){
      F[(i-1)*n+j-1]=(i^j)&1;
      int t=4-((i==1)||(i==n))-((j==1)||(j==n));
      V[(i^j)&1][t-2].push_back((i-1)*n+j-1);
    }

    bool f=1;
    
    //cerr<<"****"<<endl;
    if(Work(0))f=0;
    else if(Work(1))f=0;

    if(f)puts("Impossible");
    else{
      puts("Possible");
      REP(i,1,n+1){
        REP(j,1,n+1){
          putchar(F[(i-1)*n+j-1]==1?'R':'B');
        }
        puts("");
      }
    }
  }
    
  return 0;
}