#include <bits/stdc++.h>
using namespace std;
int n, m;
const int dx[] = {0, 0, 1, -1};
const int dy[] = {1, -1, 0, 0};
inline int cal(int u, int v){
	int ans = 0;
	for(int k = 0; k < 4; k++){
		int x = u+dx[k];
		int y = v+dy[k];
		if(x < 1 || x > n || y < 1 || y > n) continue;
		ans ++;
	}
	return ans;
}
void solve(int &cnt2, int &cnt3, int &cnt4, int n, int m){
	cnt2 = cnt3 = cnt4 = 0;
	int tot4 = 0, tot2 = 0, tot3 = 0;
	for(int i = 1; i <= n; i++){
		for(int j = 1; j <= n; j++){
			if((i+j)&1) continue;
			int ret = cal(i, j);
			if(ret == 2) tot2++;
			else if(ret == 3) tot3++;
			else if(ret == 4) tot4++;
		}
	}

	int sub = 2*n*(n-1) - m;
	if(sub & 1) 
		tot3--, cnt3++, sub--;
	{
		int can = tot3/2;
		int di_num = min(can, sub/6);
		cnt3 += di_num * 2;
		tot3 -= di_num * 2;
		sub -= di_num * 6;
	}
	if(!sub) return ;
	{
		int can = tot4;
		int di_num = min(can, sub/4);
		cnt4 += di_num;
		tot4 -= di_num;
		sub -= di_num * 4;
	}
	if(!sub) return ;
	{
		int can = tot2;
		int di_num = min(can, sub/2);
		cnt2 += di_num;
		tot2 -= di_num;
		sub -= di_num * 2;
	}

}
int main()
{
	int T;
	cin >> T;
	while(T--){
		scanf("%d%d", &n, &m);
		int tot = 2*n*(n-1);
		if(n == 1) puts("Impossible");
		else if(m == 1 || m == tot - 1){
			puts("Impossible");
		}
		else if(m > tot) {
			puts("Impossible");
		}
		else {
			puts("Possible");
			int cnt2, cnt3, cnt4;
			solve(cnt2, cnt3, cnt4, n, m);
			cout << "cnt:" << cnt2 << " "<<cnt3<<"  "<<cnt4<<endl;
			for(int i = 1; i <= n; i++){
				for(int j = 1; j <= n; j++){
					if((i+j)&1){
						putchar('B');
					}
					else{
						int ret = cal(i, j);
						if(ret == 2){
							if(cnt2) putchar('B'), cnt2--;
							else putchar('R');
						}
						else if(ret == 3){
							if(cnt3) putchar('B'), cnt3--;
							else putchar('R');
						}
						else{
							if(cnt4) putchar('B'), cnt4--;
							else putchar('R');
						}
					}
				}
				puts("");
			}
		}
	}

	return 0;
}