#include <iostream>

using namespace std;

int main() {
  cin.tie(0)->sync_with_stdio(0);
  for (int tc = (cin >> tc, tc); tc--; ) {
    int n, k; cin >> n >> k;
    string s(n * n, 'R');
    for (int i = 0; i < n; ++i)
    for (int j = i % 2; j < n; j += 2) if (i || j || n == 3) {
      int cur = 4 - (!i || i == n - 1) - (!j || j == n - 1);
      if (k >= cur && k - cur != 1) s[i * n + j] = 'B', k -= cur;
    }
    if (k == 2) s[0] = 'B', k = 0;
    else if (n == k && s[4] == 'R') s[4] = s[6], s[7] = 'B', k = 0;
    if (!k && (cout << "Possible\n"))
    for (int i = 0; i < n; ++i) cout << s.substr(i * n, n) << '\n';
    else cout << "Impossible\n";
  }
}