#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++;
				}
			}
			int C4=min(m/4,cnt4);
			int C2=min((m-C4*4)/2,cnt2);
			int C3;
			rep(i,-3,3)rep(j,-3,3){
				if(fl)continue;
				int t4=C4+i;
				int t2=C2+j;
				if(t4>=0&&t4<=cnt4&&t2>=0&&t2<=cnt2&&(m-t4*4-t2*2)%3==0){
					int t3=(m-t4*4-t2*2)/3;
					if(t3>=0&&t3<=cnt3){
						C2=t2,C4=t4,C3=t3;
						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("");
						}
						fl=1;
					}
				}
			}
		cnt2=0,cnt3=0,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++;
				}
			}
			 C4=min(m/4,cnt4);
			 C2=min((m-C4*4)/2,cnt2);
			rep(i,-3,3)rep(j,-3,3){
				if(fl)continue;
				int t4=C4+i;
				int t2=C2+j;
				if(t4>=0&&t4<=cnt4&&t2>=0&&t2<=cnt2&&(m-t4*4-t2*2)%3==0){
					int t3=(m-t4*4-t2*2)/3;
					if(t3>=0&&t3<=cnt3){
						C2=t2,C4=t4,C3=t3;
						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("");
						}
						fl=1;
					}
				}
			}
			if(!fl)puts("Impossible");
	}
}