#include <bits/stdc++.h>
using namespace std;

#define int long long
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
int _,n,k,a[2000][2000],p[9],A[9],s;
void Z(){
	cin>>n>>k;
	if(n==1){
		cout<<"Possible\nB\n";
		return;
	}
	if(k==1||k==2*n*(n-1)-1){
		cout<<"Impossible\n";
		return;
	}
	cout<<"Possible\n";
	if(n<=3){
		for(int i=0;i<(1<<(n*n));i++){
			for(int j=0;j<n;j++)
				for(int k=0;k<n;k++)
					a[j+1][k+1]=(i>>(j*n+k))&1;
			int c=0;
			for(int j=1;j<=n;j++)
				for(int k=1;k<=n;k++)
					for(int l=1;l<=n;l++)
						for(int m=1;m<=n;m++)
							c+=abs(j-l)+abs(k-m)==1&&a[j][k]!=a[l][m];
			if(c/2==k){
				for(int j=1;j<=n;j++,cout<<'\n')
					for(int k=1;k<=n;k++)
						cout<<(a[j][k]?'B':'R');
				return;
			}
		}
	}
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)
			a[i][j]=0;
	p[2]=p[3]=p[4]=A[2]=A[3]=A[4]=s=0;
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)
			if((i+j)%2==0)
				a[i][j]=4-(i==1||i==n)-(j==1||j==n),p[a[i][j]]++;
	while(A[4]<p[4]&&s+4<=k)
		A[4]++,s+=4;
	while(A[3]<p[3]&&s+3<=k)
		A[3]++,s+=3;
	while(A[2]<p[2]&&s+2<=k)
		A[2]++,s+=2;
	for(int i=max(0ll,A[2]-4);i<=min(p[4],A[2]+4);i++)
		for(int j=max(0ll,A[3]-4);j<=min(p[4],A[3]+4);j++)
			for(int l=max(0ll,A[4]-4);l<=min(p[4],A[4]+4);l++)
				if(i*2+j*3+l*4==k)
					A[2]=i,A[3]=j,A[4]=l;
	for(int i=1;i<=n;i++,cout<<'\n')
		for(int j=1;j<=n;j++)
			if(a[i][j]&&A[a[i][j]])
				cout<<'B',A[a[i][j]]--;
			else
				cout<<'R';
}
signed main() {
	ios::sync_with_stdio(0);
	for(cin>>_;_--;)
		Z();
}