#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> PII;
const double PI = acos(-1.0);
const double eps = 1e-6;
const int mod = 1e9 + 7;
const int inf = 0x3f3f3f3f;
const long long INF = 0x3f3f3f3f3f3f3f3f;
const int maxn = 1e3 + 7;
int T = 1, n, k;
vector<PII> vec[5];
int cnt[10];
char ans[maxn][maxn];

void solve(){
    cin >> n >> k;
    if(n == 3) {
        if(k == 1 || k == 11) {
            cout << "Impossible\n";
            return;
        }
        cout << "Possible" << '\n';
        if(k == 0) {
            cout << "BBB" << '\n';
            cout << "BBB" << '\n';
            cout << "BBB" << '\n';
        }
        else if(k == 2) {
            cout << "RBB" << '\n';
            cout << "BBB" << '\n';
            cout << "BBB" << '\n';
        }
        else if(k == 3) {
            cout << "BRB" << '\n';
            cout << "BBB" << '\n';
            cout << "BBB" << '\n';
        }
        else if(k == 4) {
            cout << "RBR" << '\n';
            cout << "BBB" << '\n';
            cout << "BBB" << '\n';
        }
        else if(k == 5) {
            cout << "BRB" << '\n';
            cout << "BBB" << '\n';
            cout << "RBB" << '\n';
        }
        else if(k == 6) {
            cout << "BRB" << '\n';
            cout << "BBB" << '\n';
            cout << "BRB" << '\n';
        }
        else if(k == 7) {
            cout << "BRB" << '\n';
            cout << "BBB" << '\n';
            cout << "RBR" << '\n';
        }
        else if(k == 8) {
            cout << "RBR" << '\n';
            cout << "BBB" << '\n';
            cout << "RBR" << '\n';
        }
        else if(k == 9) {
            cout << "BRB" << '\n';
            cout << "RBR" << '\n';
            cout << "BBB" << '\n';
        }
        else if(k == 10) {
            cout << "BBB" << '\n';
            cout << "BRB" << '\n';
            cout << "RBR" << '\n';
        }
        else if(k == 12) {
            cout << "ABA" << '\n';
            cout << "BAB" << '\n';
            cout << "ABA" << '\n';
        }
        return;
    }
    for(int i = 0; i < 5; i++) cnt[i] = 0, vec[i].clear();
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= n; j++) {
            ans[i][j] = 'B';
            if((i + j) % 2 == 0) {
                int tmp = ((j + 1) <= n) + ((j - 1) >= 1) + ((i + 1) <= n) + ((i - 1) >= 1);
                vec[tmp].push_back({i, j});
            }
        }
    }
    int flag = 0;
    for(int i = 0; i <= vec[3].size(); i++) {
        for(int j = 0; j <= vec[2].size(); j++) {
            if((k - i * 3 - j * 2) >= 0 && (k - i * 3 - j * 2) % 4 == 0 && (k - i * 3 - j * 2) / 4 <= vec[4].size()) {
                cnt[2] = j, cnt[3] = i, cnt[4] = (k - i * 3 - j * 2) / 4;
                flag = 1;
                break;
            }
        }
    }
    if(!flag) {
        cout << "Impossible\n";
    }
    else {
        cout << "Possible\n";
        for(int i = 0; i < cnt[2]; i++) {
            auto it = vec[2][i];
            ans[it.first][it.second] = 'R';
        }
        for(int i = 0; i < cnt[3]; i++) {
            auto it = vec[3][i];
            ans[it.first][it.second] = 'R';
        }
        for(int i = 0; i < cnt[4]; i++) {
            auto it = vec[4][i];
            ans[it.first][it.second] = 'R';
        }
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= n; j++) {
                cout << ans[i][j];
            }
            cout << '\n';
        }
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
 
    cin >> T;
    while(T--) {
        solve();
    }
    return 0;
}