#include<bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;

int cal(int mask) {
    int cnt = 0;
    for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++) {
            if (abs(i % 3 - j % 3) + abs(i / 3 - j / 3) == 1) {
                cnt += (((mask >> i) & 1) ^ (1 & (mask >> j))) != 0;
            }
        }
    }
    return cnt / 2;
}

void show(int ret) {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            int id = 3 * i + j;
            cout << (((ret >> id) & 1) ? 'R' : 'B');
        }
        cout << "\n";
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int t;
    cin >> t;

    unordered_map<int, int> ans;
    for (int msk = 0; msk < (1 << 9); msk++) {
        ans[cal(msk)] = msk;
    }

    while (t--) {
        ll n, k;
        cin >> n >> k;
        if (k == 1 || 2LL * n * (n - 1) - k == 1) {
            cout << "Impossible\n";
            continue;
        }
        if (n == 2) {
            if (k == 4) {
                cout << "Possible\n";
                cout << "RB\n";
                cout << "BR\n";
            } else if (k == 2) {
                cout << "Possible\n";
                cout << "RB\n";
                cout << "RR\n";
            }
            continue;
        }
        if (n == 3) {
            auto ret = ans[k];
            cout << "Possible\n";
            show(ret);
            continue;
        }
        vector<vector<int>> block(n + 10, vector<int>(n + 10));
        vector<pair<ll, pii>> op;
        vector<pair<ll, pii>> op3;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if ((i + j) % 2 == 0) {
                    block[i][j] = 1;
                    int cnt = 4;
                    if (i == 0 || i == n - 1)cnt--;
                    if (j == 0 || j == n - 1)cnt--;
                    op.push_back({cnt, {i, j}});
                }
            }
        }
        for (int i = 0; i < op.size(); i++) {
            if (op[i].first == 2 && i != 0) {
                swap(op[0], op[i]);
            }
        }
        for (int i = 0; i < op.size(); i++) {
            if (op[i].first == 3 && i != 1) {
                swap(op[1], op[i]);
            }
        }
        for (int i = 0; i < op.size(); i++) {
            if (op[i].first == 4 && i != 2) {
                swap(op[2], op[i]);
            }
        }
        ll num = 2LL * n * (n - 1) - k;
        ll fir = 9;
        vector<int> from(2LL * n * (n - 1) + 50, -1);
        from[2] = 0, from[3] = 1, from[4] = 2;
        from[5] = 1, from[7] = 1, from[9] = 1;
        from[6] = 2;
        for (int i = 3; i < op.size(); i++) {
            auto [cnt, pos] = op[i];
            for (int j = fir + cnt; j >= fir - 1; j--) {
                if (from[j - cnt] == -1 || j == fir)continue;
                from[j] = i;
            }
            fir += cnt;
        }
        assert(fir >= num);
        while (num > 0) {
            auto [cnt, pos] = op[from[num]];
            num -= cnt;
            block[pos.first][pos.second] = 0;
        }
        assert(num == 0);
        cout << "Possible\n";
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                cout << (block[i][j] ? 'R' : 'B');
            }
            cout << "\n";
        }

    }
    return 0;
}