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

#define ff   first
#define ss   second
#define pb   push_back

typedef long long   ll;
typedef pair<int,int>   ii;

const int N = 3e5+5;
const int mod = 1e9+7;


int v[1005][1005];

int main() { 
	#ifdef LOCAL
	freopen("in.txt","r",stdin);
	#endif
		
		
	int t; cin >> t;
	while (t--) { 
		int n; cin >> n;
		int k; cin >> k;
		if (k == 1) { 
			cout << "Impossible\n";
			continue;
		}
		if (n == 4 && k == 7) {
			cout << "Possible\n"; 
			for (int i = 0; i < n; ++i) { 
				for (int j = 0; j < n; ++j) { 
					if ((i == 1 && j == 1) || (i == 3 && j == 2)) cout << "B";
					else cout << "R";
				}
				cout << '\n';
			}
			continue;
		}
		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) { 
						int pos = j * n + k;
						if (i & (1<<pos)) v[j][k] = 1;
						else v[j][k] = 0;
					}
				}
				int cnt = 0;
				for (int j = 0; j < n; ++j) { 
					for (int k = 0; k < n; ++k) { 
						if (j + 1 < n && abs(v[j + 1][k] - v[j][k]) == 1) cnt++;
						if (k + 1 < n && abs(v[j][k + 1] - v[j][k]) == 1) cnt++; 
					}
				}
				if (cnt == k) { 
					cout << "Possible\n";
					for (int j = 0; j < n; ++j)  {
						for (int k = 0; k < n; ++k) { 
							if (v[j][k]) cout << "B";
							else cout << "R";
						}
						cout << '\n';
					}
					break;
				}
				if (i + 1 == (1<<(n*n))) {
					cout << "Impossible\n";
				}
			}
			continue;
		}
		if ((n == 4 && k <= 6) || (n > 4 && k <= 8)) { 
			for (int i = 0; i < n; ++i) 
				for (int j = 0; j < n; ++j) v[i][j] = 0;
			
			cout << "Possible\n";
			if (k > 0) {
				v[0][0] = 1;
				k -= 2;
				for (int i = 1; i + 1 < n && k; ++i, --k) v[0][i] = 1;
				for (int j = 1; j + 1 < n && k; ++j, --k) v[j][0] = 1;
			}
			for (int j = 0; j < n; ++j)  {
				for (int k = 0; k < n; ++k) { 
					if (v[j][k]) cout << "B";
					else cout << "R";
				}
				cout << '\n';
			}			  
			continue;
		}
		
		vector <ii> two, three, four;
		for (int i = 0 ; i < n; ++i) { 
			for (int j = 0; j < n; ++j) { 
				if ((i + j) % 2 == 0) { 
					v[i][j] = 1;
					if ((i == 0 && j == 0) || (i == 0 && j == n-1) || (i == n-1 && j == 0) || (i == n-1 && j == n-1)) two.pb({i,j});
					else if (i == 0 || j == 0 || i == n-1 || j == n-1) three.pb({i,j});
					else four.pb({i, j});
				}
				else v[i][j] = 0;
				
			}
		}
		
		int rst = 2 * n * (n-1) - k;
		if (rst == 1) { 
			cout << "Impossible\n";
			continue;
		}
		
		int idx3=0,idx4=0;
		
		//cout << four.size() << ' ' << three.size() << '\n';
		
		while (rst >= 1) { 
			//cout << rst << ' ' << idx4 << ' ' << idx3 << '\n';
			if (rst >= 4 && idx4 < four.size()) { 
				v[four[idx4].ff][four[idx4].ss] ^= 1;
				idx4++;
				rst-=4;
			}
			else if (rst >= 3 && idx3 < three.size()) { 
				v[three[idx3].ff][three[idx3].ss] ^= 1;
				idx3++;
				rst-=3;
			}
			else {
				if (rst == 2) { 
					v[two[0].ff][two[0].ss] ^= 1;
					rst -= 2;
				}
				else if (rst == 1) { 
					v[four[0].ff][four[0].ss] ^= 1;
					v[three.back().ff][three.back().ss] ^= 1;
					v[two[0].ff][two[0].ss] ^= 1;
					rst--;
				}
			}
		}
		
		cout << "Possible\n";
		for (int j = 0; j < n; ++j)  {
			for (int k = 0; k < n; ++k) { 
				if (v[j][k]) cout << "B";
				else cout << "R";
			}
			cout << '\n';
		}
	}
	
	return 0;
}