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

using i64 = long long;

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);

    int t;
    cin >> t;

    auto solve = [&]() {
        int n, k;
        cin >> n >> k;

        auto work = [&](auto work, int n, int k) -> vector<string> {
            if (k > n * (n - 1)) {
                auto res = work(work, n, 2 * n * (n - 1) - k);
                if (!res.empty()) {
                    for (int i = 0; i < n; i++) {
                        for (int j = 0; j < n; j++) {
                            if ((i + j) % 2 == 1) {
                                res[i][j] ^= 'R' ^ 'B';
                            }
                        }
                    }
                }
                return res;
            }
            if (k == 1) {
                return {};
            }
            vector res(n, string(n, 'R'));
            if (n == 3) {
                if (k == 0) {

                } else if (k == 2) {
                    res[0][0] = 'B';
                } else if (k == 3) {
                    res[0][1] = 'B';
                } else if (k == 4) {
                    res[1][1] = 'B';
                } else if (k == 5) {
                    res[0][1] = res[2][0] = 'B';
                } else if (k == 6) {
                    res[0][1] = res[1][0] = 'B';
                }
                return res;
            }
            if (k % 2 == 1) {
                res[0][2] = 'B';
                k -= 3;
            }
            for (int i = 1; i < n - 1; i++) {
                for (int j = 1; j < n - 1; j++) {
                    if ((i + j) % 2 == 0 && k >= 4) {
                        k -= 4;
                        res[i][j] = 'B';
                    }
                }
            }
            if (k == 2) {
                res[0][0] = 'B';
                k -= 2;
            }
            return res;
        };
        
        auto ans = work(work, n, k);

        if (ans.empty()) {
            cout << "Impossible\n";
        } else {
            cout << "Possible\n";
            for (int i = 0; i < n; i++) {
                cout << ans[i] << '\n';
            }
        }
    };

    while (t--) {
        solve();
    }
    
    return 0;
}