//Δ_N
#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef double DB;
const int N = 1111;
int n,k,a[N][N];
vector<pair<int,pair<int,int> > > v;
int main(){
	int T,i,j;
	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%2==1){
			if(k==3)
				printf("RBR\nRRR\nRRR\n");
			if(k==5)
				printf("RBR\nRRR\nBRR\n");
			if(k==7)
				printf("RBR\nRRR\nBRB\n");
			if(k==9)
				printf("RBR\nBRB\nRRR\n");
			continue;
		}
		v.clear();
		for(i=1;i<=n;i++)
			for(j=1;j<=n;j++)
				if(i%2==j%2)
					v.push_back(make_pair(4-(i==1)-(i==n)-(j==1)-(j==n),make_pair(i,j)));
		sort(v.begin(),v.end());
		reverse(v.begin(),v.end());
		for(i=1;i<=n;i++)
			for(j=1;j<=n;j++)
				a[i][j]=0;
		for(i=0;i<v.size();i++)
			if(k>=v[i].first&&k!=v[i].first+1&&!(i+1<v.size()&&v[i].first==3&&v[i+1].first==2&&k%2==0))
				a[v[i].second.first][v[i].second.second]=1,k-=v[i].first;
		for(i=1;i<=n;i++){
			for(j=1;j<=n;j++)
				printf("%c","RB"[a[i][j]]);
			printf("\n");
		}
	}
	return 0;
}