//I wrote this code 4 u today
#include <bits/stdc++.h>

template<class t> using vc = std::vector<t>;

#define nd node*
#define pnd pair<nd, nd>

//using namespace std;
typedef long long ll;

template<const ll MOD>
struct mod_mul : std::multiplies<const ll> {
    ll operator()(const ll a, const ll b) {
        return (a * b) % MOD;
    }
};


template<typename T>
inline void sort(T &a) {
    sort(a.begin(), a.end());
}

template<typename T>
inline void unique(T &a) {
    a.resize(unique(a.begin(), a.end()) - a.begin());
}

template<typename T>
inline void reverse(T &a) {
    reverse(a.begin(), a.end());
}

const ll INF = 9023372036854775808ll;
const ll MOD = 1000000007ll;

std::pair<int, int> q[4] = {{0,  1},
                            {1,  0},
                            {0,  -1},
                            {-1, 0}};

int a[3005][3005];
int max = 0;

int n, k;
vc<std::array<int, 3>> x;
bool da[3005 * 3005 * 2];

int cnt = 0;
bool gen(int i = 0) {
    if (cnt == k) {
        for (int i = 0; i < n; ++i) for (int j = 0; j < n; ++j) a[i][j] = 0;
        for (int i = 0; i < x.size(); ++i) if (da[i]) a[x[i][1]][x[i][2]] = 1;
        for (int i = 0; i < n; ++i) {
            std::string s;
            for (int j = 0; j < n; ++j) s.push_back(a[i][j] ? 'R' : 'B');
            std::cout << s << '\n';
        }
        return true;
    }
    if (i == x.size()) {
        return true;
    }
    da[i] = 1;
    cnt += x[i][0];
    if (gen(i + 1)) return true;
    da[i] = 0;
    cnt -= x[i][0];
    if (gen(i + 1)) return true;
    return false;
}

int main() {
    std::cin.tie(nullptr)->ios_base::sync_with_stdio(false);
    int t;
    std::cin >> t;
    while (t--) {
        max = 0;
        std::cin >> n >> k;
        int hex = 0;
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < n; ++j) {
                hex += i != n - 1;
                hex += j != n - 1;
            }
        }
        if (k > hex || k == hex - 1 || k == 1) {
            std::cout << "Impossible\n";
//            gen(0, 0);
        } else {
            std::cout << "Possible\n";
            x.clear();
            for (int i = 0; i < n; ++i) {
                for (int j = 0; j < n; ++j) {
                    if ((i + j + 1) % 2) {
                        if (i % (n - 1) == 0 && j % (n - 1) == 0) x.push_back({2, i, j});
                        else if (i % (n - 1) == 0 || j % (n - 1) == 0) x.push_back({3, i, j});
                        else x.push_back({4, i, j});
                    }
                }
            }
            std::sort(x.begin(), x.end());
            assert(gen());
        }
    }
}