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

#define rep(i,a,b) for(int i=a,i##end=b;i<=i##end;++i)
#define drep(i,a,b) for(int i=a,i##end=b;i>=i##end;--i)

using ll = long long;
int rd(){
	int x;
	scanf("%d",&x);
	return x;
}
int A[105][105];
int main(){
	rep(cas,1,rd()){
		int n=rd(),m=2*n*(n-1)-rd();
		bool fl=0;
		if(n<=3){
			int S=(1<<(n*n))-1;
			rep(s,0,S){
				rep(i,1,n){
					rep(j,1,n){
						if((1<<((i-1)*n+j-1))&s){
							A[i][j]=0;
						}else A[i][j]=1;
					}
				}
				int res=0;
				rep(i,1,n)rep(j,1,n){
					if(i!=n)if(A[i+1][j]==A[i][j])res++;
					if(j!=n)if(A[i][j]==A[i][j+1])res++;
				}
				if(res==m){
					fl=1;
					puts("Possible");
					rep(i,1,n){
						rep(j,1,n)printf("%c",A[i][j]?'B':'R');
						puts("");
					}
					break;
				}
			}
			if(!fl)puts("Impossible");
			continue;
		}
		int cnt2=0,cnt3=0,cnt4=0;
		rep(i,1,n)rep(j,1,n){
			if((i+j)&1){
				if((i==1||i==n)&&(j==1||j==n))
					cnt2++;
				else if((i==1||i==n)||(j==1||j==n))
					cnt3++;
				else cnt4++;
			}
		}
		rep(t2,0,cnt2)rep(t3,0,cnt3)rep(t4,0,cnt4){
			if(fl)continue;
			int C2=t2,C3=t3,C4=t4;
			if(C2*2+C3*3+C4*4==m){
				fl=1;
				puts("Possible");
				rep(i,1,n){
					rep(j,1,n){
						if((i+j)&1){
							if((i==1||i==n)&&(j==1||j==n)){
								if(C2)printf("B"),C2--;
								else printf("R");
							}
							else if(((i==1||i==n)||(j==1||j==n))){
								if(C3)printf("B"),C3--;
								else printf("R");
							}
							else {
								if(C4)printf("B"),C4--;
								else printf("R");
							}
						}else printf("B");
					}
					puts("");
				}
			}
		}
		cnt2=cnt3=cnt4=0;
		rep(i,1,n)rep(j,1,n){
			if(((i+j)&1)==0){
				if((i==1||i==n)&&(j==1||j==n))
					cnt2++;
				else if((i==1||i==n)||(j==1||j==n))
					cnt3++;
				else cnt4++;
			}
		}
		rep(t2,0,cnt2)rep(t3,0,cnt3)rep(t4,0,cnt4){
			if(fl)continue;
			int C2=t2,C3=t3,C4=t4;
			if(C2*2+C3*3+C4*4==m){
				fl=1;
				puts("Possible");
				rep(i,1,n){
					rep(j,1,n){
						if(((i+j)&1)==0){
							if((i==1||i==n)&&(j==1||j==n)){
								if(C2)printf("B"),C2--;
								else printf("R");
							}
							else if(((i==1||i==n)||(j==1||j==n))){
								if(C3)printf("B"),C3--;
								else printf("R");
							}
							else {
								if(C4)printf("B"),C4--;
								else printf("R");
							}
						}else printf("B");
					}
					puts("");
				}
			}
		}
		if(!fl){
			//if(m!=2*n*(n-1)-1&&m!=1)cout<<n<<" "<<m<<"-------"<<endl;
			puts("Impossible");
		}
	}
}