#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 main(){
	rep(cas,1,rd()){
		int n=rd(),m=rd();
		bool fl=0;
		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)puts("Impossible");
	}
}