#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define X first
#define Y second
#define SZ(a) ((int)a.size())
#define ALL(v) v.begin(), v.end()
#define pb push_back

string ans[1005];

int main() {
    ios::sync_with_stdio(0), cin.tie(0);
    int t;
    cin >> t;
    while (t--) {
        int n, k;
        cin >> n >> k;
        if (n == 1) {
            cout << "Possible\nB\n";
            continue;
        }
        if (k == 1 || k == 2 * n * (n - 1) - 1) {
            cout << "Impossible\n";
            continue;
        }
        if (n == 2) {
            if (k & 1) cout << "Impossible\n";
            else if (k == 0) cout << "Possible\nBB\nBB\n";
            else if (k == 2) cout << "Possible\nBR\nBB\n";
            else if (k == 4) cout << "Possible\nBR\nRB\n";
            else assert(0);
            continue;
        }
        for (int i = 0; i < n; ++i) ans[i] = string(n, 'B');
        if (k == 0)
            ans[0][0] = 'B';
        else if (k == 2)
            ans[0][0] = 'R';
        else if (k == 3)
            ans[0][0] = ans[0][1] = 'R';
        else if (k == 4)
            ans[0][0] = ans[n - 1][n - 1] = 'R';
        else if (k == 5)
            ans[0][1] = ans[n - 1][n - 1] = 'R';
        else {
            for (int i = 0; i < n; ++i)
                for (int j = (i & 1) ^ 1; j < n; j += 2)
                    ans[i][j] = 'R';
            int cur = 2 * n * (n - 1);
            if (k & 1) ans[0][1] = 'B', k -= 3;

            vector<pii> coord[5];
            auto pop = [&](int d) {
                cur -= d;
                auto [x, y] = coord[d].back();
                coord[d].pop_back();
                ans[x][y] = 'R';
            };

            for (int i = 0; i < n; ++i)
                for (int j = i & 1; j < n; j += 2) {
                    if ((k & 1) && i + abs(j - 1) == 1) continue;
                    int cnt = 4;
                    if (i == 0) --cnt;
                    if (i == n - 1) --cnt;
                    if (j == 0) --cnt;
                    if (j == n - 1) --cnt;
                    coord[cnt].pb(pii(i, j));
                }
            while (cur - k >= 6 && SZ(coord[3]) >= 2)
                pop(3), pop(3);
            while (cur - k >= 4 && SZ(coord[4]) >= 1)
                pop(4);
            while (cur - k >= 2 && SZ(coord[2]) >= 1)
                pop(2);
            assert(cur == k);
        }
        cout << "Possible\n";
        for (int i = 0; i < n; ++i) cout << ans[i] << "\n";
    }
}