#include<bits/stdc++.h>
using namespace std;
const int N = 1005;
int ans[N][N];
int n, k, prek;
bool check(int x, int y) {
    if(x < 1 || y < 1 || x > n || y > n) return false;
    return true;
}

int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};

int change(int x, int y) {
    int pre = 0;
    int nxt = 0;
    for(int i = 0; i < 4; i++) {
        if(check(x + dx[i], y + dy[i]) && ans[x + dx[i]][y + dy[i]] != ans[x][y]) pre++;
        if(check(x + dx[i], y + dy[i]) && ans[x + dx[i]][y + dy[i]] == ans[x][y]) nxt++;
    }
    return nxt - pre;
}

void verify() {
    int res = 0;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++) {
            if(i != 1 && ans[i][j] != ans[i - 1][j]) res++;
            if(j != 1 && ans[i][j] != ans[i][j - 1]) res++;
        }
    assert(res == prek);
}

void print(int opt) {
    cout << "Possible\n";
    for (int i = 1; i <= n; i++, cout << "\n")
        for (int j = 1; j <= n; j++) {
            if (opt) ans[i][j] ^= ((i + j) & 1);
            if (ans[i][j] == 0) cout << "R";
            else cout << "B";
        }
    verify();
}

int main() {
    #ifdef _DEBUG
        freopen("data.txt", "r", stdin);
        freopen("data.out", "w", stdout);
    #endif
    ios::sync_with_stdio(false);
    cin.tie(0);
    int T;
    cin >> T;
    while(T--) {
        cin >> n >> k;
        prek = k;
        for (int i = 1; i <= n; i++)
            for (int j = 1; j <= n; j++) ans[i][j] = 0;
        if(n <= 3) {
            int flag = 1;
            for(int i = 0; i < (1 << (n * n)); i ++) {
                for (int i = 1; i <= n; i++)
                    for (int j = 1; j <= n; j++) ans[i][j] = 0;
                int res = 0;
                for (int j = 0; j < n; j++)
                    for (int k = 0; k < n; k++) {
                        int num = j * n + k;
                        int d = ((i >> num) & 1);
                        if(d) res += change(j + 1, k + 1);
                        ans[j + 1][k + 1] = d;
                    }
                if(res == k) {
                    flag = 0;
                    print(0);
                    break;
                }
            }
            if(flag == 1) cout << "Impossible\n";
            continue;
        }
        if(k == 1) {
            cout << "Impossible\n";
            continue;
        } else {
            if(k <= n * (n - 1)) {
                if (k == 0) {
                    print(0);
                    continue;   
                } else {
                    int flag = 1;
                    for (int i = 1; i <= n && flag; i += 2) {
                        for (int j = 1; j <= n && flag; j++) {
                            int d = change(i, j);
                            int cnt = 0;
                            if(d == 0) continue;
                            if(d == 3) {
                                ans[1][n - 1] ^= 1;
                                ans[1][n - 2] ^= 1;
                                cnt = 2;
                                d = 1;
                            } 
                            else if(d == 2 && !(i == 1 && j == 1)) {
                                ans[1][n - 1] ^= 1;
                                cnt = 1;
                                d = 1;
                            }
                            ans[i][j] ^= 1;
                            k -= d;
                            if(cnt == 2 && k >= 1) {
                                ans[1][n - 2] ^= 1;
                                k--;
                                cnt--;
                            }
                            if(cnt == 1 && k >= 1) {
                                ans[1][n - 1] ^= 1;
                                k --;
                                cnt --;
                            }
                            if(k == 0) {
                                print(0);
                                flag = 0;
                            }
                        }
                    }
                }
            } else {
                k = 2 * n * (n - 1) - k;
                if (k == 0) {
                    print(1);
                    continue;   
                } else {
                    int flag = 1;
                    for (int i = 1; i <= n && flag; i += 2) {
                        for (int j = 1; j <= n && flag; j++) {
                            int d = change(i, j);
                            int cnt = 0;
                            if(d == 0) continue;
                            if(d == 3) {
                                ans[1][n - 1] ^= 1;
                                ans[1][n - 2] ^= 1;
                                cnt = 2;
                                d = 1;
                            } 
                            else if(d == 2 && !(i == 1 && j == 1)) {
                                ans[1][n - 1] ^= 1;
                                cnt = 1;
                                d = 1;
                            }
                            ans[i][j] ^= 1;
                            k -= d;
                            if(cnt == 2 && k >= 1) {
                                ans[1][n - 2] ^= 1;
                                k--;
                                cnt--;
                            }
                            if(cnt == 1 && k >= 1) {
                                ans[1][n - 1] ^= 1;
                                k --;
                                cnt --;
                            }
                            if(k == 0) {
                                print(1);
                                flag = 0;
                            }
                        }
                    }
                }
            }
        }
    }
}