#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define int long long
void dbg_out() { cerr << endl; }
template <typename H, typename... T>
void dbg_out(H h, T... t) { cerr << ' ' << h; dbg_out(t...); }
#define dbg(...) { cerr << #__VA_ARGS__ << ':'; dbg_out(__VA_ARGS__); }
using ll = long long;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

const int MAXN = 2e5 + 5;
const int MOD = 1e9+7; //998244353;
const int INF = 0x3f3f3f3f;
const ll INF64 = ll(4e18) + 5;

vector<pair<int,int>> d4 = { {1,0}, {0,1}, {-1,0}, {0,-1}};

void solve(){
	int n,k;
	cin >> n >> k;
	for(int op = 0; op < 2; op++){
		vector<vector<int>> ans(n, vector<int>(n));
		auto calc = [&](int i, int j){
			int at = 0;
			for(int w = 0; w < 4; w++){
				int ni = i + d4[w].fi;
				int nj = j + d4[w].se;
				if(ni >= 0 && nj >= 0 && ni < n && nj < n){
					at += ans[ni][nj] != ans[i][j];
				}
			}
			return at;
		};

		auto print = [&](){
			cout <<"Possible\n";
			for(int i = 0; i < n; i++){
				for(int j = 0; j < n; j++){
					cout << (ans[i][j] == 0 ? 'B' : 'R');
				}
				cout << '\n';
			}
		};
		int sum = 0;
		auto flip = [&](int i, int j){
			sum -= calc(i,j);
			ans[i][j] ^= 1;
			sum += calc(i,j);
		};
		int fi = -1;
		int fj = -1;
		for(int i = 0; i < n; i++){
			for(int j = 0; j < n; j++){
				if( (i+j+op)%2 == 0){
					flip(i,j);
					if(sum > k){
						fi = i;
						fj = j;
						flip(i,j);
					}
					if(sum == k){
						break;
					}
				}
			}
			if(sum == k || fi != -1) break;
		}
		if(fi == -1 && sum != k){
			continue;
		}
		if(sum == k){
			print();
			return;
		}
		vector<pair<int,int>> vals;
		for(int dx = -1; dx <= 1; dx++){
			for(int dy = -1; dy <= 1; dy++){
				int ni = fi+dx;
				int nj = fj+dy;
				if(ni >= 0 && nj >= 0 && ni < n && nj < n){
					vals.pb({ni,nj});
				}
			}
		}
		int sz = vals.size();
		for(int mask = 0; mask < (1<<n); mask++){
			for(int w = 0; w < sz; w++) if( (mask>>w)&1){
				auto [i,j] = vals[w];
				flip(i,j);
			}
			if(sum == k){
				print();
				return;
			}
			for(int w = 0; w < sz; w++) if( (mask>>w)&1){
				auto [i,j] = vals[w];
				flip(i,j);
			}

		}
	}
	cout << "Impossible\n";
}

signed main(){

	ios::sync_with_stdio(false); cin.tie(NULL);

	int t = 1;
	cin >> t;
	while(t--){
		solve();
	}

	return 0;
}

/*
Makefile:
CXXFLAGS=-Wall -Wextra -Wshadow -g -pedantic -fsanitize=address,undefined -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUGPEDANTIC -std=gnu++17
 */