#include<iostream>
#include<cstring>
#include<vector>
using namespace std;
using LL = long long;

int main(){

#ifdef LOCAL
    freopen("data.in", "r", stdin);
    freopen("data.out", "w", stdout);
#endif

    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(0);

    int T;
    cin >> T;
    while(T--){
        int n, k;
        cin >> n >> k;
        
        auto solve = [&](int target){
            int sum = n * (n - 1) * 2;
            vector<vector<int> > a(n, vector<int>(n));
            vector<pair<int, int> > v[5];
            for(int i = 0; i < n; i++){
                for(int j = 0; j < n; j++){
                    if ((i + j) % 2 == target){
                        a[i][j] = 1;
                        int t = 4 - (i == 0 || i == n - 1) - (j == 0 || j == n - 1);
                        v[t].push_back({i, j});
                    }
                }
            }
            for(int i = 0; i <= v[2].size(); i++){
                for(int j = 0; j <= v[3].size(); j++){
                    int need = sum - k - 2 * i - 3 * j;
                    if (need % 4 == 0 && need >= 0 && need / 4 <= v[4].size()){
                        for(int t = 0; t < i; t++){
                            auto [x, y] = v[2][t];
                            a[x][y] = 0;
                        }
                        for(int t = 0; t < j; t++){
                            auto [x, y] = v[3][t];
                            a[x][y] = 0;
                        }
                        for(int t = 0; t < need / 4; t++){
                            auto [x, y] = v[4][t];
                            a[x][y] = 0;
                        }
                        cout << "Possible" << '\n';
                        for(int i = 0; i < n; i++){
                            for(int j = 0; j < n; j++){
                                cout << (a[i][j] ? 'R' : 'B');
                            }
                            cout << '\n';
                        }
                        return true;
                    }
                }
            }
            return false;
        };

        if (!solve(0) && !solve(1)){
            cout << "Impossible" << '\n';
        }
    }

}