#include <bits/stdc++.h>

using namespace std;

#define io ios_base::sync_with_stdio(false); cin.tie(0); cerr.tie(0)
#define iter(v) v.begin(), v.end()
#define SZ(v) int(v.size())
#define pb emplace_back
#define mp make_pair
#define ff first
#define ss second

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#ifdef zisk
void debug(){cerr << "\n";}
template<class T, class ... U>
void debug(T a, U ... b){cerr << a << " ", debug(b...);}
template<class T> void pary(T l, T r){
	while(l != r) cerr << *l << " ", l++;
	cerr << "\n";
}
#else
#define debug(...) void()
#define pary(...) void()
#endif
template<class A, class B>
ostream& operator<<(ostream& o, pair<A, B> p){
	return o << '(' << p.ff << ',' << p.ss << ')';
}

int main(){
	io;

	int T;
	cin >> T;
	while (T--) {
		int n, k;
		cin >> n >> k;
		bool flip = 0;
		if (k > n * (n-1)) {
			flip = 1;
			k = 2*n*(n-1) - k;
		}
		vector<vector<int>> grid(n, vector<int>(n, 0));	
		for (int i = 1;i < n;i += 2) {
			for (int j = 0;j < n;j++) grid[i][j] = 1;
		}
		if (k == 1) {
			cout << "Impossible\n";
		} else {
			int diff = n * (n-1) - k;	
			for (int i = 2;i < n;i+=2) {
				for (int j = 0;j < n;j++) {
					int val = 0;
					if (i < n - 1) val = 1;
					if (j > 0) val++;
					if (j == n-1) val++;
					if (diff >= val) {
						diff -= val;
						grid[i][j] = 1;
					}
				}
			}
			for (int j = 0;j < n;j++) {
				int val = 0;
				if (j > 0) val++;
				if (j == n-1) val++;
				if (diff >= val) {
					diff -= val;
					grid[0][j] = 1;
				}
			}
			assert(diff == 0);
			cout << "Possible\n";
			if (flip) {
				for (int i = 0;i < n;i++) {
					for (int j = 0;j < n;j++) {
						if ((i+j)&1) grid[i][j] ^= 1;
					}
				}
			}
			for (int i = 0;i < n;i++) {
				for (int j = 0;j < n;j++) cout << (grid[i][j]?'R':'B');
				cout << "\n";
			}
		}
	}

}