#include<iostream>
#include<cstdio>
#include<queue>
#define maxn 1005
#define pii pair<int,pair<int,int>>
#define m_p make_pair
#define a_f first
#define a_s second
using namespace std;
int T,n,k,d[maxn][maxn]; char a[maxn][maxn];
priority_queue<pii> q;
int main(){
	scanf("%d",&T); while(T--){
		scanf("%d%d",&n,&k); if(k==1||k==2*n*(n-1)-1){printf("Impossible\n"); continue;} printf("Possible\n");
		if(n==3&&k==7){printf("BRB\nBBB\nRBR\n"); continue;}else if(n==3&&k==9){printf("BRB\nRBB\nBRB\n"); continue;}
		else if(n==3&&k==3){printf("BRB\nBBB\nBBB\n"); continue;}else if(n==3&&k==5){printf("BRB\nBBB\nRBB\n"); continue;}
		if(k==3){
			for(int i=1;i<=n;i++) for(int j=1;j<=n;j++)
				{if(i==1&&j==2) printf("R"); else printf("B"); if(j==n) printf("\n");} continue;
		}else if(k==5){
			for(int i=1;i<=n;i++) for(int j=1;j<=n;j++)
				{if(i==1&&(j==1||j==3)) printf("R"); else printf("B"); if(j==n) printf("\n");} continue;
		}else 
		while(!q.empty()) q.pop(); for(int i=1;i<=n;i++) for(int j=1;j<=n;j++) if((i+j)%2==0){
			a[i][j]='R'; if((i==1||i==n)&&(j==1||j==n)) d[i][j]=2;
			else if(i==1||j==1||i==n||j==n) d[i][j]=3; else d[i][j]=4;
			q.push(m_p(d[i][j],m_p(i,j)));
		}else a[i][j]='B'; k=2*n*(n-1)-k;
		while(k>0){
			pii top=q.top(); q.pop(); while(k-top.a_f==1||k-top.a_f<0) 
			top=q.top(),q.pop();
			k-=top.a_f; a[top.a_s.a_f][top.a_s.a_s]='B';
		} for(int i=1;i<=n;i++) for(int j=1;j<=n;j++){printf("%c",a[i][j]); if(j==n) printf("\n");}
	} return 0;
}