#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const int N = 1e3 + 10;
char a[N][N];

bool getbit(int mask, int bit) {
    return mask & (1 << bit);
}

bool solve(int n, int k) {
    if (k == 0) {
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n; j++) {
                a[i][j] = 'B';
            }
        }
        return true;
    } else if (n <= 4) {
        for (int mask = 0; mask < (1 << (n * n)); mask++) {
            for (int i = 1; i <= n; i++) {
                for (int j = 1; j <= n; j++) {
                    if (getbit(mask, (i - 1) * n + j - 1)) {
                        a[i][j] = 'R';
                    } else {
                        a[i][j] = 'B';
                    }
                }
            }
            int cur = 0;
            for (int i = 1; i <= n; i++) {
                for (int j = 1; j <= n; j++) {
                    if ((i + j) % 2 == 0) {
                        if (i >= 2 && a[i][j] != a[i - 1][j]) cur++;
                        if (i <= n - 1 && a[i][j] != a[i + 1][j]) cur++;
                        if (j >= 2 && a[i][j] != a[i][j - 1]) cur++;
                        if (j <= n - 1 && a[i][j] != a[i][j + 1]) cur++;
                    }
                }
            }
            if (cur == k) {
                return true;
            }
        }
        return false;
    } else {
        int cnt[5] = {};
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n; j++) {
                if ((i + j) % 2 == 0) {
                    int cur = 0;
                    if (i >= 2) cur++;
                    if (i <= n - 1) cur++;
                    if (j >= 2) cur++;
                    if (j <= n - 1) cur++;
                    cnt[cur]++;
                }
            }
        }
        int trg[5] = {};
        bool ok = false;
        for (int i = 0; i <= cnt[4]; i++) {
            for (int j = 0; j <= cnt[2]; j++) {
                if (k >= 4 * i + 2 * j && (k - 4 * i - 2 * j) % 3 == 0 && (k - 4 * i - 2 * j) <= cnt[3]) {
                    trg[2] = j;
                    trg[3] = (k - 4 * i - 2 * j) / 3;
                    trg[4] = i;
                    ok = true;
                }
            }
        }
        if (ok) {
            for (int i = 1; i <= n; i++) {
                for (int j = 1; j <= n; j++) {
                    if ((i + j) % 2 == 0) {
                        int cur = 0;
                        if (i >= 2) cur++;
                        if (i <= n - 1) cur++;
                        if (j >= 2) cur++;
                        if (j <= n - 1) cur++;
                        if (trg[cur]) {
                            a[i][j] = 'R';
                            trg[cur]--;
                        } else {
                            a[i][j] = 'B';
                        }
                    } else {
                        a[i][j] = 'B';
                    }
                }
            }
            return true;
        } else {
            return false;
        }
    }
}

void foo() {
    int n = 3;
    int cnt[5] = {};
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            if ((i + j) % 2 == 0) {
                int cur = 0;
                if (i >= 2) cur++;
                if (i <= n - 1) cur++;
                if (j >= 2) cur++;
                if (j <= n - 1) cur++;
                cnt[cur]++;
            }
        }
    }
    set<int> s;
    for (int i = 0; i <= cnt[4]; i++) {
        for (int j = 0; j <= cnt[3]; j++) {
            for (int k = 0; k <= cnt[2]; k++) {
                for (int l = 0; l <= cnt[1]; l++) {
                    for (int x = 0; x <= cnt[0]; x++) {
                        s.insert(4 * i + 3 * j + 2 * k + 1 * l + 0 * x);
                    }
                }
            }
        }
    }
    cout << 0 << " " << 2 * n * (n - 1) << "\n";
    for (int i : s) {
        cout << i << " ";
    }
    cout << "\n";
    exit(0);
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
#ifdef LOCAL
    freopen("input.txt", "r", stdin);
#endif

    // foo();

    int t;
    cin >> t;
    while (t--) {
        int n, k;
        cin >> n >> k;
        if (solve(n, k)) {
            cout << "Possible\n";
            for (int i = 1; i <= n; i++) {
                for (int j = 1; j <= n; j++) {
                    cout << a[i][j];
                }
                cout << "\n";
            }
        } else {
            cout << "Impossible\n";
        }
    }

#ifdef LOCAL
    cout << "\nTime elapsed: " << double(clock()) / CLOCKS_PER_SEC << " s.\n";
#endif
}