#include<bits/stdc++.h>
using namespace std;
const int maxn = 2e6 + 5;
const int step[4][2] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
int T, n, k, dp[maxn], pre[maxn][5], mp[1005][1005];
vector<pair<int, int> > pos[5];
pair<int, int> val[maxn];
int Solve(int x) {
	int tot = 0;
	for(int i = 2; i <= 4; ++ i)
		pos[i].clear();
	for(int i = 1; i <= n; ++ i) {
		for(int j = 1; j <= n; ++ j) {
			mp[i][j] = (i + j) % 2;
			if((i + j) % 2 == x) {
				int cnt = 0;
				for(int k = 0; k < 4; ++ k) {
					int ni = i + step[k][0], nj = j + step[k][1];
					if(ni >= 1 && ni <= n && nj >= 1 && nj <= n)
						++ cnt;
				}
				pos[cnt].push_back({i, j});
				tot += cnt;
			}
		}
	}
	// cerr << tot << endl;
	if(k == 1 || k == tot - 1 || k > tot)
		return 0;
	for(int i = 1; i <= tot; ++ i)
		dp[i] = 0;
	dp[0] = 1, pre[0][2] = pre[0][3] = pre[0][4] = 0;
	int w = 0;
	for(int i = 2; i <= 4; ++ i) {
		int all = pos[i].size();
		for(int bit = 1; bit <= all; bit <<= 1) {
			val[++ w] = {bit, i};
			all -= bit;
		}
		if(all) {
			val[++ w] = {all, i};
		}
	}
	for(int i = 1; i <= w; ++ i) {
		int s = val[i].first * val[i].second;
		for(int j = tot; j >= s; -- j) {
			if(!dp[j] && dp[j - s]) {
				dp[j] = 1;
				for(int k = 2; k <= 4; ++ k)
					pre[j][k] = pre[j - s][k];
				pre[j][val[i].second] += val[i].first;
			}
		}
	}
	if(dp[tot - k]) {
		for(int i = 2; i <= 4; ++ i) {
// cerr << i << " " << pre[tot - k][i] << endl;
			for(int j = 0; j < pre[tot - k][i]; ++ j) {
				mp[pos[i][j].first][pos[i][j].second] ^= 1;
			}
		}
		puts("Possible");
		for(int i = 1; i <= n; ++ i, puts(""))
			for(int j = 1; j <= n; ++ j)
				putchar(mp[i][j] ? 'R' : 'B');
		return 1;
	}
	return 0;
}
int ni[20], nj[20], all;
int main() {
	for(scanf("%d", &T); T --; ) {
		scanf("%d%d", &n, &k);
		if(n <= 4) {
			all = 0;
			for(int i = 1; i <= n; ++ i) {
				for(int j = 1; j <= n; ++ j) {
					ni[all] = i;
					nj[all] = j;
					all ++;
				}
			}
			int f = 0;
			for(int j = 0; j < 1 << all; ++ j) {
				for(int a = 1; a <= n; ++ a)
					for(int b = 1; b <= n; ++ b)
						mp[a][b] = 0;
				for(int k = 0; k < all; ++ k)
					if(j >> k & 1)
						mp[ni[k]][nj[k]] = 1;
				int now = 0;
				for(int a = 1; a <= n; ++ a)
					for(int b = 1; b <= n; ++ b)
						if((a + b) % 2 == 0) {
							for(int k = 0; k < 4; ++ k) {
								int na = a + step[k][0], nb = b + step[k][1];
								if(na >= 1 && na <= n && nb >= 1 && nb <= n && mp[a][b] != mp[na][nb])
									++ now;
							}
						}
				if(now == k) {
					f = 1;
					puts("Possible");
					for(int i = 1; i <= n; ++ i, puts(""))
						for(int j = 1; j <= n; ++ j)
							putchar(mp[i][j] ? 'R' : 'B');
					break;
				}
			}
			if(!f) {
				puts("Impossible");
			}
			continue;
		}
		if(Solve(0)) {
			continue;
		}
		if(Solve(1)) {
			continue;
		}
		puts("Impossible");
	}
}