#include <bits/stdc++.h>
using namespace std;

const int N = 1005;
int a[N][N],n,k;

void work()
{
	scanf("%d %d",&n,&k);
	// printf("%d %d\n",n,k);
	if(n==1)
	{
		if(k==0)
		{
			cout<<"Possible\n";
			cout<<"B\n";
		}
		else cout<<"Impossible\n";
	}
	else
	{
		for(int i=1;i<=n;i++) for(int j=1;j<=n;j++) a[i][j]=0;
		vector<pair<int,int>> v[5];
		for(int i=1;i<=n;i++) for(int j=1;j<=n;j++)
		{
			if((i+j)%2==0)
			{
				int cnt=0;
				if(i==1||i==n) cnt+=1;
				else cnt+=2;
				if(j==1||j==n) cnt+=1;
				else cnt+=2;
				v[cnt].emplace_back(i,j);
			}
		}
		int mid=n*(n-1);
		if(k>mid)
		{
			k=mid*2-k;
			for(int i=1;i<=n;i++) for(int j=1;j<=n;j++) if((i+j)%2==0) a[i][j]^=1;
		}
		if(n==3&&k==3)
		{
			a[1][2]^=1;
			k-=3;
		}
		else if(n==3&&k==5)
		{
			a[1][2]^=1,a[3][3]^=1;
			k-=5;
		}
		else if(n==4&&k==12)
		{
			a[1][1]^=1,a[2][2]^=1,a[3][3]^=1,a[4][4]^=1;
			k-=12;
		}
		else
		{
			while(k>=6)
			{
				if(!v[4].empty())
				{
					auto [x,y]=v[4].back(); v[4].pop_back();
					a[x][y]^=1,k-=4;
				}
				else if(!v[3].empty())
				{
					auto [x,y]=v[3].back(); v[3].pop_back();
					a[x][y]^=1,k-=3;
				}
				else
				{
					assert(!v[2].empty());
					auto [x,y]=v[2].back(); v[2].pop_back();
					a[x][y]^=1,k-=2;
				}
			}
			if(k==5)
			{
				assert(!v[3].empty());
				{
					auto [x,y]=v[3].back(); v[3].pop_back();
					a[x][y]^=1,k-=3;
				}
				assert(!v[2].empty());
				{
					auto [x,y]=v[2].back(); v[2].pop_back();
					a[x][y]^=1,k-=2;
				}
			}
			else if(k==4)
			{
				assert(!v[4].empty());
				auto [x,y]=v[4].back(); v[4].pop_back();
				a[x][y]^=1,k-=4;
			}
			else if(k==3)
			{
				assert(!v[3].empty());
				auto [x,y]=v[3].back(); v[3].pop_back();
				a[x][y]^=1,k-=3;
			}
			else if(k==2)
			{
				assert(!v[2].empty());
				auto [x,y]=v[2].back(); v[2].pop_back();
				a[x][y]^=1,k-=2;
			}
		}
		if(k==0)
		{
			printf("Possible\n");
			for(int i=1;i<=n;i++)
			{
				for(int j=1;j<=n;j++) putchar("BR"[a[i][j]]);
				putchar('\n');
			}
		}
		else printf("Impossible\n");
	}
}

int main() {
#ifdef xay5421
	freopen("a.in","r",stdin);
#endif
	int T;
	scanf("%d", &T);
	while (T--) {
		work();
	}
	return 0;
}