#include<bits/stdc++.h>
#define io ios::sync_with_stdio(false);
#define off cin.tie(0), cout.tie(0);
#define all(x) x.begin(),x.end()
#define inf 0x3f3f3f3f3f3f3f3f
#define MAXN 1000007
#define int long long

using namespace std;

const int mod = 1e9 + 7;
const double eps = 1e-7;

int t = 1, n, m, x, k;
int sum2, sum3, sum4, cnt2, cnt3, cnt4;

char g[1010][1010];

void run() {
    cin >> n >> k;
    if (n == 3) {
        if (k == 1 || k == 11) {
            cout << "Impossible\n";
            return ;
        }
        cout << "Possible\n";
        for (int i = 1; i <= 3; i++) for (int j = 1; j <= 3; j++) g[i][j] = 'B';
        if (k == 2) {
            g[1][1] = 'R';
        } else if (k == 3) {
            g[1][2] = 'R';
        } else if (k == 4) {
            g[2][2] = 'R';
        } else if (k == 5) {
            g[1][1] = 'R';
            g[3][2] = 'R';
        } else if (k == 6) {
            g[1][2] = 'R';
            g[3][2] = 'R';
        } else if (k == 7) {
            g[1][1] = g[1][3] = g[3][2] = 'R';
        } else if (k == 8) {
            g[1][2] = g[2][1] = g[3][3] = 'R';
        } else if (k == 9) {
            g[1][2] = g[2][1] = g[3][2] = 'R';
        } else if (k == 10) {
            g[1][1] = g[1][3] = g[3][1] = g[2][2] = 'R';
        } else if (k == 12) {
            g[1][1] = g[1][3] = g[3][1] = g[3][3] = g[2][2] = 'R';
        }
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                cout << g[i][j];
            }
            cout << '\n';
        }
        return ;
    }
    sum2 = (n % 2 ? 4 : 2), sum3 = (n - 2) / 2 * 4, sum4 = ((n - 2) * (n - 2) + 1) / 2;
    // cout << sum2 << ' ' << sum3 << ' ' << sum4 << '\n';
    int flag = 0;
    for (int cnt3 = 0; cnt3 <= sum3; cnt3++) {
        for (int cnt2 = 0; cnt2 <= sum2; cnt2++) {
            if ((k - 2 * cnt2 - 3 * cnt3) % 4 == 0 && (k - 2 * cnt2 - 3 * cnt3) / 4 >= 0 && (k - 2 * cnt2 - 3 * cnt3) / 4 <= sum4) {
                int cnt4 = (k - 2 * cnt2 - 3 * cnt3) / 4;
                flag = 1;
                cout << "Possible\n";
                int _2 = cnt2, _3 = cnt3, _4 = cnt4;
                // cerr << _2 << ' ' << _3 << ' ' << _4 << '\n';
                for (int i = 1; i <= n; i++) {
                    for (int j = 1; j <= n; j++) {
                        if ((i + j) % 2 == 0) {
                            // cerr << _2 << ' ' << _3 << ' ' << _4 << '\n';
                            if ((i == 1 && j == 1) || (i == 1 && j == n) || (i == n && j == 1) || (i == n && j == n)) {
                                if (_2) {
                                    _2--;
                                    g[i][j] = 'R';
                                } else g[i][j] = 'B';
                            } 
                            else if (i == 1 || i == n || j == 1 || j == n) {
                                if (_3) {
                                    _3--;
                                    g[i][j] = 'R';
                                } else g[i][j] = 'B';
                            } else {
                                if (_4) {
                                    _4--;
                                    g[i][j] = 'R';
                                } else g[i][j] = 'B';
                            }
                        } else {
                            g[i][j] = 'B';
                        }
                    }
                }
                for (int i = 1; i <= n; i++) {
                    for (int j = 1; j <= n; j++) {
                        cout << g[i][j];
                    }
                    cout << '\n'; 
                }
            }
            if (flag) break;
        }
        if (flag) break;
    }
    if (flag == 0) {
        cout << "Impossible\n";
    }
}

signed main () {
    io off
    cin >> t;
    while (t--) {
        run();
    }
}