#include <iostream>
#include <vector>
#include <cmath>
#include <tuple>
#include <set>

using namespace std;
using vb = vector<bool>;
using vvb = vector<vb>;

vvb find_ans(const int n, const int k) {
    if (n == 3)
        if (k == 5)
            return {{1, 0, 0},
                    {0, 0, 1},
                    {0, 0, 0}};
        else if (k == 7)
            return {{1, 0, 0},
                    {0, 0, 1},
                    {1, 0, 0}};
    if (k < 0 || k == 1 || k == n * (n - 1) * 2 - 1 || k > n * (n - 1) * 2)
        return {};
    for (int o = 0; o < 2; ++o) {
        vvb ans(n, vb(n));
        int K = k;
        for (int i = 1; (K >= 4 && K != 5) && i < n - 1; ++i)
            for (int j = 1; (K >= 4 && K != 5) && j < n - 1; ++j)
                if ((i + j) % 2 == o) {
                        K -= 4;
                        ans[i][j] = true;
                    }
        for (int i = 0; (K >= 3 && K != 4) && i < n; ++i) {
            int dob = (i == 0 || i == n - 1) ? 1 : n - 1;
            for (int j = 0; (K >= 3 && K != 4) && j < n; j += dob)
                if ((i + j) % 2 == o)
                    if (!((i == 0 || i == n - 1) && (j == 0 || j == n - 1))) {
                        K -= 3;
                        ans[i][j] = true;
                    }
        }
        for (int i = 0; (K >= 2) && i < n; i += n - 1)
            for (int j = 0; (K >= 2) && j < n; j += n - 1)
                if ((i + j) % 2 == o) {
                    K -= 2;
                    ans[i][j] = true;
                }
        if (K == 0) return ans;
    }
    return {};
}

void solve_test() {
    int n, k;
    cin >> n >> k;
    vvb ans = find_ans(n, k);
    if (ans.size()) {
        cout << "Possible\n";
        for (const vb &a: ans) {
            for (bool b: a)
                cout << (b ? 'R' : 'B');
            cout << '\n';
        }
    } else
        cout << "Impossible\n";
}

void solve_tests() {
    int t;
    cin >> t;
    for (int i = 0; i < t; ++i)
        solve_test();
}

int main() {
#ifdef HOME
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    solve_tests();
}