#pragma GCC optimize("Ofast","unroll-loops","omit-frame-pointer","inline")
#pragma GCC option("arch=native","tune=native","no-zero-upper")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2")
#pragma GCC optimize(3)
#include<bits/stdc++.h>
#define N 1010
#define rep(i,l,r) for(int i=l;i<=r;i++)
#define drep(i,r,l) for(int i=r;i>=l;i--)
using namespace std;
int rd() {
	int res=0,f=1;char ch=getchar();
	while(ch<'0'||ch>'9') {if(ch=='-') f*=-1;ch=getchar();}
	while(ch>='0'&&ch<='9') res=(res<<1)+(res<<3)+(ch^48),ch=getchar();
	return res*f;
}
int T,n,K;
bool a[N][N];
int gets(int i,int j) {
	int res=0;
	if(i>1) res+=a[i-1][j]!=a[i][j];
	if(i<n) res+=a[i+1][j]!=a[i][j];
	if(j>1) res+=a[i][j-1]!=a[i][j];
	if(j<n) res+=a[i][j+1]!=a[i][j];
	return res;
}
int main() {
	T=rd();
	while(T--) {
		n=rd(),K=rd();
		rep(i,1,n) rep(j,1,n) a[i][j]=0;
		rep(i,1,n) {
			for(int j=(i&1)+1;j<=n;j+=2) {
				a[i][j]=1;
				int tmp=gets(i,j);
				if(K>=tmp) a[i][j]=1,K-=tmp;
				else a[i][j]=0;
			}
		}
		if(K) rep(i,1,n) rep(j,1,n) {
			int tmp=gets(i,j);a[i][j]^=1;
			if(K==gets(i,j)-tmp) break;
			a[i][j]^=1;
		}
		if(!K) {
			puts("Possible");
			rep(i,1,n) {
				rep(j,1,n) if(a[i][j]) printf("B");else printf("R");
				puts("");
			}
		}
		else puts("Impossible");
	}
	return 0;
}