#include <bits/stdc++.h>
using namespace std;
const int N=1010;
inline int read()
{
	char ch;
	while((ch=getchar())<'0'||ch>'9');
	int res=ch-'0';
	while((ch=getchar())>='0'&&ch<='9') res=res*10+ch-'0';
	return res;
}
int n,K,a[N][N];
bool dfs(int x,int y,int now)
{
	if(now>K) return 0;
	if(x==n)
	{
		if(now==K)
		{
			for(int i=0;i<n;printf("\n"),i++)
				for(int j=0;j<n;j++)
				printf("%c",a[i][j]?'R':'B');
			return 1;
		}
		return 0;
	}
	a[x][y]=0;
	int tmp=now+(x&&a[x-1][y]!=a[x][y])+(y&&a[x][y-1]!=a[x][y]);
	if(y==n-1?dfs(x+1,0,tmp):dfs(x,y+1,tmp)) return 1;
	a[x][y]=1;
	tmp=now+(x&&a[x-1][y]!=a[x][y])+(y&&a[x][y-1]!=a[x][y]);
	if(y==n-1?dfs(x+1,0,tmp):dfs(x,y+1,tmp)) return 1;
	return 0;
}
int val(int x,int y){
	return 4-(x==0||x==n-1)-(y==0||y==n-1); 
}
int cnt[5],ans[5];
bool dfs2(int now,int rst)
{
	if(now==2)
	{
		if(!(rst&1)&&2*cnt[2]>=rst) 
		{
			ans[2]=rst/2;
			return 1;
		}
		return 0;
	}
	for(int i=rst/now;i>=0;i--)
		if(dfs2(now-1,rst-i*now)) 
		{
			ans[now]=i;
			return 1;
		}
	return 0;
}
int main()
{
	int T=read();
	while(T--)
	{
		n=read(),K=read();
		if(K==1||K==2*(n-1)*n-1) printf("Impossible\n");
		else
		{
			printf("Possible\n");
			if(n<=3) dfs(0,0,0);
			else
			{
				cnt[2]=cnt[3]=cnt[4]=0;
				for(int i=0;i<n;i++)
					for(int j=0;j<n;j++)
					{
						a[i][j]=0;
						if(!((i+j)&1)) cnt[val(i,j)]++;
					}		
				dfs2(4,K);
				for(int i=0;i<n;printf("\n"),i++)
					for(int j=0;j<n;j++)
					if(!((i+j)&1)&&ans[val(i,j)]) ans[val(i,j)]--,printf("R");
					else printf("B");
			}
		}
	}
	return 0;
}