#include <bits/stdc++.h>
using namespace std;
#define int long long

vector<int> make(int a, int b, int c, int x) {
    // From a 2s, b 3s, c 4s --> Make the sum x.
    // Return {-1, -1, -1} if not possible.
    for(int A = 0; A <= a; A++) {
        for(int B = 0; B <= b; B++) {
            int y = x - 2 * A - 3 * B;
            if(y < 0 || y % 4 != 0 || (y / 4) > c) continue;
            return {A, B, y / 4};
        }
    }
    return {-1, -1, -1};
}

int32_t main() {
    cin.tie(0)->sync_with_stdio(0);

    int t;
    cin >> t;
    while(t--) {
        int n, k;
        cin >> n >> k;
        if(n == 1) {
            cout << "Possible" << '\n';
            cout << "R" << '\n';
            continue;
        }
        int up = 2ll * n * (n - 1);
        if(k == 1 || k == up - 1) {
            cout << "Impossible" << '\n';
            continue;
        }
        // cout << n << ' ' << k << endl;
        if(n == 3 && k == 5) {
            cout << "Possible" << '\n';
            cout << "RRB\nBBB\nRBB\n";
            continue;
        }
        if(n == 3 && k == 7) {
            cout << "Possible" << '\n';
            cout << "RRB\nBBB\nRBR\n";
            continue;
        }
        int two, three, four;
        vector<vector<bool>> grid(n, vector<bool>(n));
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < n; j++) {
                if((i + j) % 2 == 0) {
                    grid[i][j] = true;
                }
            }
        }
        if(n % 2 == 0) {
            two = 2;
            three = 4*((n/2) - 1);
            four = n*n/2 - two - three;
        } else {
            two = 4;
            three = 2 * (n - 3);
            four = n*n/2 + 1 - two - three;
        }
        auto config = make(two, three, four, up - k);
        if(config[0] == -1) {
            assert(n % 2 == 1);
            two = 0;
            three = 2*(n - 1);
            four = n*n/2 - two - three;
            config = make(two, three, four, up - k);
            for(int i = 0; i < n; i++) for(int j = 0; j < n; j++) grid[i][j] = !grid[i][j];
        }
        // cout << config[0] << ' ' << config[1] << ' ' << config[2] << '\n';
        assert(config[0] != -1);
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < n; j++) {
                if(grid[i][j] == false) continue;
                bool corner = (i == 0 || i == n - 1) && (j == 0 || j == n - 1);
                bool side = (i == 0 || i == n - 1 || j == 0 || j == n - 1);
                if(corner) {
                    if(config[0] > 0) {
                        grid[i][j] = false;
                        config[0]--;
                    }
                } else if(side) {
                    if(config[1] > 0) {
                        grid[i][j] = false;
                        config[1]--;
                    }
                } else {
                    if(config[2] > 0) {
                        grid[i][j] = false;
                        config[2]--;
                    }
                }
            }
        }
        cout << "Possible" << '\n';
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < n; j++) {
                cout << (grid[i][j] ? 'B' : 'R');
            }
            cout << '\n';
        }
    }

}