#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define rep(i, j, k) for (ll i = j; i <= k; ++i)
#define per(i, j, k) for (ll i = j; i >= k; --i)
#define closeSync            \
    ios::sync_with_stdio(0); \
    cin.tie(0);              \
    cout.tie(0)
#define pii pair<ll, ll>
#define fi first
#define se second
const ll INF = 0x3f3f3f3f;
const ll mod = 1e9 + 7;

void solve() {
    ll n, k;
    cin >> n >> k;
    ll lim = 2 * n * (n - 1);
    if (k == 1 || k == lim - 1) {
        cout << "Impossible\n";
        return;
    }
    cout << "Possible\n";
    vector<vector<int>> mp(n + 1, vector<int>(n + 1, 0));
    if (n <= 4) {
        for (int sta = 0; sta < (1 << n * n); ++sta) {
            for (int j = 0; j < n * n; ++j) {
                bool f = (sta >> j) & 1;
                mp[j / n + 1][j % n + 1] = f;
            }
            int cnt = 0;
            rep(i, 1, n) {
                rep(j, 2, n) {
                    if (mp[i][j] != mp[i][j - 1]) cnt++;
                    if (mp[j][i] != mp[j - 1][i]) cnt++;
                }
            }
            if (cnt == k) {
                rep(i, 1, n) {
                    rep(j, 1, n) {
                        if (mp[i][j] == 1)
                            cout << "R";
                        else
                            cout << "B";
                    }
                    cout << "\n";
                }
                return;
            }
        }
    }
    int f = 0;
    if (k > lim / 2) {
        k = k - lim / 2;
        f = 1;
    }
    if (k & 1) {
        mp[1][3] = 1;
        k -= 3;
    }
    if (k % 4) {
        mp[1][1] = 1;
    }
    k /= 4;
    int x = 2, y = 2;
    while (k--) {
        mp[x][y] = 1;
        y += 2;
        if (y >= n) {
            x++;
            if (x & 1) {
                y = 3;
            } else {
                y = 2;
            }
        }
    }

    if (f) {
        rep(i, 1, n) {
            rep(j, 1, n) {
                if (i + j & 1) mp[i][j] ^= 1;
            }
        }
    }
    rep(i, 1, n) {
        rep(j, 1, n) {
            if (mp[i][j] == 1)
                cout << "R";
            else
                cout << "B";
        }
        cout << "\n";
    }
}

int main() {
    closeSync;
    int T;
    cin >> T;
    while (T--)
        solve();
    return 0;
}